Make pry-byebug more extendable

[zenspider]

https://gist.github.com/zenspider/8049ee1bc6b994e2098508d278183dbc

The first paragraph of the above linked code extends Byebug::PryProcessor#perform to be extendable... namely it extracts the list of valid names to a constant and tests for inclusion there. That way plugins can add to that and use pry-byebug mechanisms.

The rest of the code adds a new command bt that prints a backtrace w/o ruby or gems paths as an example. You're welcome to any/all of it.