New Feature: add account whitelist

add a whitelist to the prx -> pmp import, only import pieces/series where account is in the whitelist.
Can be written as `PRXAccountWhitelist.wont_be :allow?, 10001` which also won't peg to a specific `true` or `false` value but worth with truthy or falsy values. Not sure if it's better, just wanted to make sure this was clear.

This might also be the situation to bring in an `assert`, potentially.