Possible feature: Ability to teach PPI about certain bits of syntax, keywords, at runtime

[wchristian]

Talking with @shadowcat-mst about Damian Conway's PPR he brought up the fact that there's a bunch of modules that are currently able to extend Perl's base set of syntax with new keywords, with the sanction of the Perl5Porters.

At the same time PPI is currently not able to parse these in the same manner, as it does not know how to recognize added syntax.

It would be useful if users of PPI could extend its parsing to let it know when custom keywords will be in the code to be parsed.