Import/Export functionality

[Booligoosh]

Suggested by @owenvoke in #27.

Users should have a way to import and export their feeds and settings. Things to consider:

Export

• Allow exporting in a JSON format specific to 1Feed as well as OPML for other feed readers
• Consider whether to have 1 export that includes feeds/folders as well as settings or whether to separate them
• Consider allowing the user to export individual folders as 1Feed JSON and/or generic OPML
• Include an export_version field so I can handle data structure changes in future

  Import

• Consider whether to make importing an "adding" thing or an "overwriting" thing, or both
• Make sure there's strong confirmation from the user before importing if going the overwriting route
• Need to make sure to fully overwrite folders on import, rather than merging the object and leaving old folders
• Consider allowing import from OPML and other common feed reader export types. This would use the adding method rather than overwriting.