Add a refresh form for feeds.

This is step 1 for decoupling feed refreshing from page loads.
Refresh feeds every 30 minutes in a cron instead of on page load.

Fixes #76
Filter .xml files when selecting an OPML file.

Fixes #113
Merge pull request #118 from eaburns/dev/refresh

Dev/refresh
Merge pull request #119 from eaburns/dev/acceptxml

Filter .xml files when selecting an OPML file.