Support for theming

[dbuenzli]

For historical reasons odoc theming has been implemented in odig.

The way it works there is described at the cli level in odig's manual here; see also odig odoc-theme --help. The theme install structure is here (you can forget about the OCaml manual bit).

This functionality should be moved to odoc so that it becomes easier for people to generate documentation with a theme that is accessible to them. For example it's currently quite difficult to change it in a build system like dune.

Besides it would be nice if any js file in the theme directory could be added to the pages generated by odoc like highlight.pack.js is. This will enable nice experimentations, nowadays that .js files are also excuted when your browse them over the file:// protocol.