handle use feature '...' up to lexical_subs (5.18).

Hi,

This adds support for

    use feature '...';

for all features up to 'lexical_subs', which requires 5.18.0.

the 'unicode_strings' feature was introduces in 5.12, but only declared "full functionality" in 5.14, but I made this require 5.14 rather than 5.12.

Cheers,
Neil