templatekw: add p1/p2 keywords which switches the current ctx
This is just an example of mappingdict.
I have no idea what should be displayed as {p1|json}. Currently it is an
empty dict since {'ctx'} isn't displayable, which is clearly useless and
should be changed later.
templater: add wrapper for a single template mapping
This can be used to nest template mappings without inserting a sequence-like
layer. See the next patch for example.