prepend optional specified libs to @INC

Implemented the `-I` switch to load used modules from an arbitrary path. Helpful to shorten the otherwise lengthy test idiom while developing DuckPAN. Uses [Getopt::Long](http://perldoc.perl.org/Getopt/Long.html) from core, with "bundling" to mimic Perl invocation.

Requested in issue https://github.com/duckduckgo/p5-app-duckpan/issues/19.

Should be added to the documentation somewhere, wasn't sure what's the right place.