Fix: Avoid compiling Peep.Plug when Plug module is not present

[scudelletti]

Hello there and Happy new year! 🎉

I noticed plug is marked as optional in the mix.exs however the lib doesn't compile when plug is not present.

==> peep
Compiling 18 files (.ex)
    error: module Plug.Conn is not loaded and could not be found
    │
 54 │   import Plug.Conn
    │   ^
    │
    └─ lib/plug.ex:54:3: Peep.Plug (module)


== Compilation error in file lib/plug.ex ==
** (CompileError) lib/plug.ex: cannot compile module Peep.Plug (errors have been logged)

I wrapped Peep.Plug around a conditional(if Code.ensure_loaded?(Plug) so it doesn't try to create that module when plug is not present.

An alternative would be to remove the optional part from the mix.exs.

[rkallos]

Thank you! <3