Added "Read more" support for intro filter

Link template is controlled by INTRO_LINK variable in conf.py
If INTRO_LINK=='', link isn't produced.
Updated documentation with INTRO_LINK conf.py option
Fixed intro filter to handle lists and other tags correctly
Updated intro filter test to handle added args correctly
Merge pull request #95 from nevkontakte/master

"Read more" link support for "intro" filter.