This required some refactoring in MultiCommand and ExpressionCommand.

Whole part about processing flags was refactored. Code related to flags
from DCommand was moved into MultiCommand and reused from there in all
"multi" commands.

New attribute: ExpressionCommand.last_argument. Should be set to True if
command is using the last argument as something other than todo ID or
part of an expression (as in case of postpone or pri). By default set to
False.

New method: MultiCommand.execute_not_multi(): wrapper for functionality
not connected with todo IDs (like `topydo edit` or `topydo edit -d`).

New attribute: MultiCommand.multi_mode: Should be set to True if user
wants to operate on multiple todo items. True by default.

New method: MultiCommand.get_todos_from_expr(): gets todo IDs based on
expression and puts it in MultiCommand.todos.

EditCommand._open_in_editor() now accepts only one argument - file to
editing. It handles now also OSError exceptions. It is used also to
opening todo.txt and done.txt.

MultiCommand.get_todos() doesn't accept anymore todo IDs as an argument.
Instead it's using inherited Expression.last_argument` to decide upon
where to find todo IDs.