Move format parsing logic to ListFormatParser

This way we can safely remove `PrettyPrinterIndentFilter` and output
length is still properly maintained by `ListFormatParser`.

ListFormatParser is now the core of PrettyPrinterFormatFilter, but can
also be easily used in another areas. Whole functionality of
PrettyPrinterAlignFilter was also moved into it.

Main advantage of having separate parser for list_format is possibility
of initializing it and resolving placeholders only once. Previous
implementation did that with each Todo object.

- From now on literal `%` signs always have to be escaped with `\` (`\%`),
even inside conditional braces.
- `%%` won't work anymore.
- Nonexistent placeholders (for example`%&`) will be expanded to empty
  strings (`''`).

This change fixes also some bugs:
- inappropriate output width when \t was used without %S
  (example: `%p{ }\t%{ }p`)
- curly braces inside conditional braces (example: `%{{}p{}}`)

Minor: recognize literal `\t` in list_format config option and in `-F`.

They mostly deal with corner cases (small terminals, dodgy texts in the
prefix/suffix).

Also prepare the test for the new syntax where a literal % will be
written as \% instead of %%.

Update tests for ls_format

2015-11-06 as freeze_time date was introduced in #75 so let's stick with
it also here.

This was missing according to coverage.

It has been replaced by the %k keyword when using formatting (`ls -F`).

Conflicts:
	setup.py

Add tests for list_format

Also add freezegun to dependencies -  we use it for mocking datetime
objects.

Users should rather use %k in list_format if they want to avoid hidden
tags on the output.

Coveralls doesn't support branch coverage, while codecov does.

A couple more tests

Thanks to @mruwek for reporting.

The project/context, tag and URL matching patterns are not aware of any color codes that are already prepended to a word. This affects items that appear at the start of a line.

Given a todo string where a priority color was already added:

\x1b[0mkey:value

Then the match would turn it into something like this:

\x1b[\x1b[1;32m0mkey:value\x1b[0m

So you get an ugly 0m prefix to the key:value text, because ] is a word boundary and 0m are non-whitespace characters that end up as being part of the key.

Report and suggested fix by @mruwek. It became apparent with `ls -F %k`, which puts tags at the start of a line, a not so common case that slipped through when writing the colorization code.

In case a URL starts at the start of the line, don't insert an additional space.

Reported by @mruwek.

The comment is not stripped from the value by the ConfigParser, so

    identifiers = text ; foo

didn't work.

Although the documention says that inline comments are allowed:
https://docs.python.org/2/library/configparser.html?highlight=comment

Add customizable right alignment in list_format

shlex.split() in python2 doesn't work very well with Unicode as
proven already in #66, so we're using ushlex when running on python2.

Specified minimum version of arrow and forced pulling
backports.shutil_get_terminal_size for python versions < 3.3.

User can now mark place in list_format from where he wants to start
right-aligning remaining content. Tab character (\t) is used as marker.

Added also 'top' alias example to topydo.conf which utilizes this
feature and implements #52.

Minor:
    - remove `- 1` from statements setting line width
    - move strip_braces() to topydo.lib.ListFormat and rename it to
      strip_placeholder_braces()

fixup strip_braces

For some weird reason, the aliases code block was not properly formatted on Github.

Failure was obscured by other JSON output failures under Cygwin.

This should have caught the bug that was fixed in a92411f7.