There was a restriction in place that a file should contain the same
number of todo items before and after an edit. This is now removed.

Reason for change is that I lost a lot of edits to a long list of items
and removing one single item from the list. The editor exits and as a
result topydo refused to process it, losing all the changes.

Another reason is that we have `revert` now. In case you severely mess
up, you can always revert.

This reuses the logic that the backup functionality uses to determine
whether a command is read-only or not. In that case, prompt mode doesn't
trigger a warning for ls* commands when the backing todo file was
changed.

Save `edit` result only when tempfile was modified

topydo.Command.get_subcommand() is one of the critical parts of topydo
so we really have to be sure that it's functioning properly.

Also move resolving aliases logic to a function.

Aliases can now be defined in 'aliases' section of topydo config file.

Examples:

[aliases]
showall = ls -x
purge = del -f
ical = ls -f ical

Sanitize defaults in topydo.lib.Config._Config

This may break the tests in pending PR #52 .

mtimes of tempfile prior to editing and after leaving the editor are now
compared before changing anything in todolist. If tempfile wasn't
modified, todolist (and todo file) is left untouched and user gets
appropriate error message.

Using defaults as a parameter passed to the constructor of
configparser.ConfigParser results in a presence of **all** options
inside every section. It's very convenient in many use-cases, but not
very much with our configuration model.

After this change only relevant options are present in each section, so:

`topydo.lib.Config.config().has_option('topydo', 'priority_colors')`

will now return `False` as 'priority_colors' resides in 'colorscheme'
section and not in 'topydo'. Before this change it would return `True`.

Introduce backup feature and `revert` command

`topydo revert` will search for backup corresponding with current state
of todo file, and use it to recover previous state. Output will show to
user results of which command had been reverted. If no suitable backup
is found (for example if user edited todo file recently with other
application), no action is taken and user is notified on the output
about lack of corresponding backup in backup file.

Backups containing whole todolist and archive can be now saved after
execution of each "read-write" command. Furthermore this change creates
base for eventual "revert" command.

Backups are safely stored and indexed in our own JSON-based format which
is compatible with python2.x and python3.x. We also use zlib compression
to minimize size of backup file. Path of the backup file is always
relative to the todo file, so backups from different todo files won't
mix up.

User can configure number of stored backups with new config option -
"backup_count". Any positive number will tell topydo to store that very
number of backups. Setting "backup_count" to 0 will completely turn off
backup functionality.

This will be used to recreate TodoList from a list of Todo objects.

Style Changes

Fix Wheel building

Evaluate conditional requirements at install time rather than compile time.

First line should end with a period

No blank lines allowed after function docstring

1 blank line required after class docstring

Allows monitoring to see how close we are to supporting these on these
platforms.

Sorted using `isort`. Sorted alphabetically, with sections for standard
library, third-party, and then internal imports.

comparison to None should be 'if cond is None:' or 'if cond is not None:'

statement ends with a semicolon

the backslash is redundant between brackets

line too long (> 79 characters)

blank line at end of file

too many blank lines

expected 2 blank lines, found 1 (above module level defined classes and
functions)