Add odoc support-files-targets

[aantron]

With this PR:

$ odoc support-files-targets
odoc.css
highlight.pack.js

The command has no options.

I assume this output is suitable for consumption. cc @dbuenzli, @rgrinberg (though I will/can make a PR to Dune to use this).

Resolves #174.

See also #153 (comment).

[dbuenzli]

The command has no options.

I think this needs an -o DIR option. This is not about listing the support files it's about which file paths the odoc support-files will write.

[aantron]

Ok, it now has the same options as support-files:

$ odoc support-files-targets --output-dir foo
foo/odoc.css
foo/highlight.pack.js

$ odoc support-files-targets --output-dir foo --without-theme
foo/highlight.pack.js

The only problem is that the code fails if the output directory doesn't exist. I'll undo that next, but I want to check whether the output and command line are otherwise reasonable.

[dbuenzli]

Thanks ! Looks to me exactly what is needed.