twitter: implemented filter based group chats

Filter group chats allow for the ability to read the tweets of select
users without actually following the users, and/or track keywords or
hashtags. A filter group chat can have multiple users, keywords, or
hashtags. These users, keywords, or hashtags can span multiple group
chats. This allows for rather robust filter organization.

The underlying structure for the filters is based on linked list, as
using the glib hash tables requires >= glib-2.16 for sanity. Since the
glib requirement of bitlbee is only 2.14, linked list are used in order
to prevent an overly complex implementation.

The idea for this patch was inspired by Artem Savkov's "Twitter search
channels" patch.

In order to use the filter group chats, a group chat must be added to
the twitter account. The channel room name is either follow:username,
track:keyword, and/or track:#hashtag. Multiple elements can be used by
separating each element by a semicolon.
Oh I see it now. Not so much memory leaks but a `free()` of the return value of `bee_user_by_handle()`. Okay