Add a function which triggers creation/refresh of the Kiva font cache

Warming up the Kiva font cache is a very expensive operation (several seconds). Most code will only trigger it when drawing text, which is generally gives a bad user experience.

We should instead provide a function in the public API which triggers creation of the cache if it is not yet built. Currently `kiva.fonttools.font_manager.default_font_manager` does this _as a side-effect_, but it is not part of the public API. **I don't think it should become public**, but I do think another function should exist for managing the cache.