GET /subs now supports extensions
Added extension check for OPML
Server now responds with appropriate data type
Added OPML info to docs
OPML export via both Accept header and path extensions is now possible
OPML import now supported
Simplified Content-Type checking code
Changed reference style to string in utils
OPML import now supports multipart req. Cleaned up code.
Support for multipart request on frontend. Not active.
Merge OPML into develop