Support multiple -o/--outfile flags in a single invocation

[ryan-crabbe]

Summary

• Change -o/--outfile to accept multiple values via action="append", so users can write multiple output formats in one invocation
• Each outfile infers its renderer from the file extension (unless -r is explicitly set, which applies to all)
• No -o or single -o behavior is unchanged
• Adds tests for multiple outfiles, single outfile, explicit renderer with multiple outfiles, and unrecognized extension error

Closes #422

[joerick]

I like the idea, but I don't like the change to existing behaviour - previously the -r option took precidence over the file extension to determine format.

Perhaps one way to fix this would be to also allow multiple -r commands, so that,

• if there's one, we use the current behaviour (all outfiles use that renderer)
• if there's more than one, each -r affects the following -o outfile arguments.
• when no -r is passed, we use the file extension.

The other (maybe more punative way) would be to just disallow -r when using multiple outfiles. This might be preferable if the code complexity to acheive the above is too high.

It would also be good to make sure we're documenting the outfile extension inferring behaviour in the --help output - as we increasingly use file extension to determine format, we need to make sure the docs and helptext explain this (and make the various formats discoverable).