bee-chat: create temporary users for unknown chat participants

The imcb_chat_msg() function is unable to send messages to a chat with
a user who was not previously added. This function should allow for the
sending of messages with users who are not added. This is suitable for
protocols which are sending messages to a chat from random users or a
large amount of users which join and part frequently.
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.