Template logic for install instructions

It might be nice to have some template logic setup for R vs Python installation instructions. Would this cause problems in dual R/Python bootcamps with multiple rooms? I can put a PR together which implements the template logic for R, Python, both if that would be useful.

**Proposal:**

1. Add a template metadata variable `bootcamplanguage` to `/index.html` with `r`, `python` options.
2. Add some template logic to `_includes/setup.html` to select appropriate blocks based on value of `bootcamplanguage` so that setup instructions reflect the language being taught.

Do we need a `both` option for bootcamps that have both, or does/should each room get its own page?
And 'hg' (for Mercurial).