Add support for Pkl files

[gordonbondon]

Pkl is an open source configuration language.

Additions:

• .pkl file extension for config files.
• PklProject file for project configuration.

[asottile]

extensions must be unambiguous to be accepted

[gordonbondon]

Is this related to possible conflict with python pickle format? Those do not seem to declare pkl as an extension and suggest pickle as a file extension https://docs.python.org/3.7/library/pickle.html#examples

Not aware of other systems using pkl extension so far.