Support loading data at runtime, or by configuring a different location for a preinstalled version

[jellybob]

No license lawyering please - unless you're someone able to speak on behalf of freedesktop.org your interpretation of the GPL isn't going to add anything to this conversation

See rails/rails#41750 and minad#97 for background. History on this issue is at jellybob#1.

In order to cause minimal impact on existing users of the mimemagic gem, particularly people using Rails, I'm going to have it load MIME types from a preinstalled version of the Freedesktop MIME types database, rather than bundling one with the gem. This will require having a copy of that either installed with your distribution, or obtained in some other way. The availability of that will be checked at build time.