diff should infer intended output format from suffix

[cmungall]

It would be more consistent with other behavior if diff took the default diff output format from the file suffix.

The current behavior is a bit confusing, as the default "plain" is almost idiomatic markdown; for example, doing an arbitrary diff on 2 versions of envo with -o diff.md and no --format md gives lines like this:

- SubClassOf(<ENVO:01000197>[broadleaf forest biome] ObjectSomeValuesFrom(<RO:0000056>[participates in] <ENVO:01001827>[climactic ecological succession]))

If the diff consists only of deletions, then this looks like a markdown list, the <ENVO:01000197>[broadleaf forest biome] looks kind of like a broken markdown link.

This could be considered a breaking change which we don't do, in which case I think just an additional note of caution in the docs should be sufficient to close this (I can do this if this is the preferred action)

[jamesaoverton]

I think this would be a good change. The risk of breakage seems small to me.