* support for :perl, :perlfile, :perldo and perleval()
* document that the perl provider doesn't currently work on Windows
* document that the perl legacy interface is now also supported
* added perleval() documentation
* import legacy perl interface tests
* only perl 5.22+ is supported
* healtcheck: use g:perl_host_prog if its set instead
using just 'perl' isn't correct as it may not be the version requested.
ditto for 'cpanm', rather go through 'App::cpanminus' to find the latest
perl version