Add more filter expressions: ~=, !=, !~=

[Andrey Vlasovskikh <andrey.vlasovskikh@gmail.com>]

At this moment only exact `=` is supported in filters.

I propose adding the following constructs:

* `~=` for "contains"
* `!~=` for "does not contain"
* `!=` for "is not equal to"

Some examples of filters that use these constructs are provided below.

All the tickes by John Smith that are not fixed:

    $ hg ilist -p from~=Smith -p state!=fixed

All the tickets of John Smith by specifying his email:

    $ hg ilist -p to=~smith@example.com

All the tickets of high priority (i. e. of `critical` priority or not
prioritized ones), notice that the logical "or" should be used for joining the
same properties:

    $ hg ilist -p priority=critical -p priority!=''