add Pry::Slop.

for a couple of different reasons we decided to remove the runtime
dependency on the rubygem `slop`, and add Pry::Slop, a copy of
leejarvis/slop@master.

Pry::Slop doesn't include commands.rb though since pry never uses that
feature. the reason we decided to do this are:

- the new slop won't support the APIs pry use, so we would need to consider
  upgrading.

- pry has more flexibility when it comes editing or making updates to option
  parsing code. it can be tailored to the needs of pry.

- a (rubygem) runtime dependency for what was two files seems excessive
  when put alongside pry's other dependencies.