Removed unused features and renamed core custom tags/attributes

Renamed:
c-if —> if
c-var —> var
c-require —> require
c-for —> for
c-def —> def
c-invoke —> invoke
c-attrs —> attrs
c-include —> include
c-with —> with
c-for-each —> for-each

Removed:
c-replace
c-content
c-strip
c-choose
c-when
c-otherwise
Renamed "context" to "out"