[RFC] Multi-OS support

For neovim/neovim#1538.
When, say, `python-client`'s `.travis.yml` calls this like `eval $(curl ...this script...) nightly-x64`, `nightly-x64` is appended to the code from the file. Before this PR, this would just execute the `nightly-x64` function.

Unfortunately, dashes are not allowed in function names in `dash` (the Ubuntu default shell), so I renamed the function names to use underscores.

To not introduce yet another breaking change (and because dashes look nicer, IMO), when `deps-x64` is now passed, it is appended to `_call_function`, thus making it the function parameter `$1`. Inside `_call_function`, dashes are replaced with underscores, and so for this example, the `deps_x64` function would be executed.

Hope that makes sense.