ImageMagick: Make Perl support optional instead of forcibly turned off

The "without-perl" option has been in this formula for 4 years with a comment saying "I couldn't make this compile".

Testing on 10.8.6 and 10.9 with this option removed, it compiles just fine.

Perl support is disabled by default in case there is a genuine issue with compilation on some setups that has been lost to the mists of time - however you now have the option to enable it if you wish.

Also added are caveats explaining how the Perl Modules and the version of ImageMagick itself must be kept in sync.