Hosting the docs not a the root

[jachin]

Hello, thanks for putting this project together.

I've run into a bit of an issue, putting the generated docs, on a URL that is not the root.

For instance if I want the docs to live at `http://my.url/docs/ that doesn't seem to be working.

If I'm understanding correctly your "How it works" section of the README this part jumped out at me:

It is a fork of the elm/package.elm-lang.org repo that takes a flag that specifies which URL path the frontend app is mounted at.

I looked a bit at your fork of elm/package.elm-lang.org and your elm-doc branch, and I noticed the mountedAt = "/".

This is a feature request I guess, but do you think it's possible to make that configurable? Maybe as an option to the elm-doc tool called --mounted-at.

I'm looking into "hacky" way to do it now, with a search a replace on the generated docs. If I come up with something that works I'll post it here.