Infer format from extension for new entries.
Infer format from extension for new entries.

If a collection has no format specified, we try to infer the format from
the file extension when reading. This commit also allows us to infer the
format from the `extension` set in the config, so that we can still
create the correct format when making a new file.

Aloomaio/netlify-cms,netlify/netlify-cms,Aloomaio/netlify-cms,netlify/netlify-cms,netlify/netlify-cms,netlify/netlify-cms