LFG Filter for Premade Groups, v3

It's great to have built-in Premade Group finder (aka LFG internally) except all the groups are mixed together and you need to carefully scan list to find that one raid or difficulty you've interested in or use TEXT based filter and risk missing groups you'd want.

== Features ==
* Sorts all groups by raid/difficulty, so, say, all you heroic groups for some specific raid are together.
* Allows you to filter search lists to only results you want, removing everything else from list.

== Filtering expressions ==
To filter results, enter a search expression into filter editbox on group search screen starting with =. Both text and expression will be used if you have some text before =. Expressions are constructed from variables that hold info on each found group and use Lua syntax and if you're unfamiliar with it, a short summary will be presented below. If you make an error while writing expression it will be printed in chat when you try to search and result list will be empty.

=== Supported variables ===
Logical values:
 * highmaul - group is running Highmaul raid.
 * normal - group is running normal difficulty.
 * heroic - group is running heroic difficulty.
 * my_server - group is running on my server. Useful for searching for trash runs or guild runs if you wish to join them later in mythic.

String values,  all strings are automatically converted to lower case to simplify your searches. You can use string functions listed below:
 * name - group's name - that big yellow text.
 * comment - longish comment entered by group leader, that place where people usually put "bring curve" or something.

Number values:
 * ilvl - minimum ilevel required by group.
 * defeated - amount of bosses defeated.
 * members - people already in group.
 * tanks/healers/damagers - amount of players in tank/healer/damager role.
 * warriors/deathknights/paladins/monks/priests/shamans/druids/rogues/mages/warlocks/hunters - amount of players of respective class.

For each variable that have plural name there's also a copy with singular name for convenience. So both "mages" and "mage" or "tanks" and "tank" will work, for example.

=== Short summary of Lua syntax ===
 * and, or, not - logical operators. "highmaul and normal" will find Highmaul normal raids, "highmaul and (normal or heroic)" will find Highmaul normal and heroic and will hide mythic.
 * == - equal, ~= - not equal, comparison for everything.
 * < > <= >= - comparison for numbers.
 * :find - a string function, true if requested substring is present.
 * :match - a string function, can be used for more complex searches with regular expressions. Look it up in Lua manual.
 * () can be used to group and define precedence.

=== Examples ===
* =highmaul and heroic and defeated == 0
** Fresh Highmaul Heroic.

* =highmaul and ((normal and (name:find("imp") or defeated == 6)) or (heroic and defeated == 1))
** Find Highmaul raid groups, either normal with 6 bosses defeated or "imp" mentioned in name, or heroic with just one boss defeated.

* =highmaul and name:find("trash") and name:find("mythic") and my_server
** Find Highmaul mythic trash runs on your server.

* =members > 3
** Find groups with at least 3 members. It will skip some groups that's just starting, but will also reliably hide recruit ads and other spam.

== Change log ==
2014-01-24 v3
* Support both plural and singular names for variables that hold amount of something.
* Add variables for number of players of specific class.

2014-01-21 v2
* Complete filtering rework: now works with expressions entered directly into group search filter field.

2014-11-12 v1
* Initial implementation: sort + activity ID search.