Add caching to file-system, and a setting to allow configuration of time between caching

[Oblongmana]

Per discussion on #8 - would be for users to be able to configure time between re-caching. Though the recent doc shakeup at sf did break the plguin, this is unlikely to happen very frequently (knock on wood), so we can probably presume the doc index remains relatively stable - so a 7 day cache expiry is probably sensible. If a situation like the recent doc shakeup arises again, we can probably force a cache refresh with a patch release somehow, or at the very least warn users with a release notes file through package control