Bypass font enumeration if a DTCoreTextFontOverrides.plist file is provided

This fixes a performance issue. Retrieving all the system fonts is quite
slow; and although this is done on a background thread, the parsing will
block until the overrides table is built.

So, if a DTCoreTextFontOverrides.plist is found, consider it canonical
and don't build the overrides table dynamically.
Merge pull request #459 from kemenaran/font-enumeration-override

Bypass font enumeration if a DTCoreTextFontOverrides.plist file is provided