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.

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.

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

Failure was obscured by other JSON output failures under Cygwin.

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

Now it conforms to the normal todo.txt CLI output.

Online coverage reports via coveralls.io

"Modules should have short, all-lowercase names. Underscores can be used in
the module name if it improves readability."

Fix unicode values in aliases config in python2

Apparently shlex and ConfigParser.read() don't work well with Unicode
input in python2.

Fix help in prompt mode

`help` is not a real subcommand and without arguments it is of NoneType,
so we need to properly handle it.

This fixes the issue that `ls -F "%p %s"` doesn't work, because the
double quotes weren't interpreted properly.

Reported by @mruwek.

Suppose format string:

 ls_format = %s [%T]

The result would be:

 Test todo item [just now

The final bracket is missing, because it was matched as part of the
placeholder.

This change will only match the closing bracket when the opening bracket
is also present.

This will make sure that %[T] in the format string gets replaced with
the replacing text.

Add %S - truncated version of %s

Properly split alias options wrapped in quotes

If %S is used in list_format, PrettyPrinterFormatFilter will now check
if final result fits on the screen. If not, it will truncate it
accordingly and only then return.

© by @MinchinWeb for `topydo.lib.Utils.get_terminal_size`

%h stands for: "due %D, starts in/started %S"
%H stands for: "%C, %h"

Also reorder %k and %K to maintain consistency in code.

Adjust `PrettyPrinterFormatFilter` to achieve current `topydo ls` output (and more)

Also fix Unicode error for %K when tag contains special characters.

It shows the first todo item to be done, optionally with a filter
applied:

topydo next +Chores @Home