Support loading modules from another library path

Useful if `catmandu` is in your PATH and custom Catmandu:: modules (importer, exporter, fixes..) are not in PERL_LIB but in some local library path. The patch allows this:

    catmandu -I lib convert MyFormat to JSON < file.myformat

where `Catmandu/Importer/MyFormat.pm` is located in `lib`. My current nasty workaround is

    perl -Ilib `which catmandu` convert MyFormat to JSON < file.myformat

It's covered by a unit test. Looks like Catmandu::CLI was not being tested before.