Added the assign tag.

As part of this, filter definitions had to be moved to core (since both tags
and ouputs use filters). Also, the parser can now parse filters directly. To
avoid cyclical referencing, core is responsible for hte FilterLookup, but
the main liquid project is responsible for registering filters with core.
updated readme for new filter structure
Added capture tag. Easier to test via the template "integration" tests then
to unit test directly.