[RFC] Add nightly build.

[Travis build](https://travis-ci.org/fwalch/bot-ci/jobs/37064879) is currently running. If it works, the release will show up at https://github.com/fwalch/neovim/releases/nightly. Had a buffer overflow on Travis during `make install` (generating helptags) with build type `MinSizeRel`, trying again with default.

Currently, this will update the `nightly` tag to point to the commit hash the tarball was built from. WIP because of that and because I wanted to create a 32-bit nightly as well.

@myitcv would this help with neovim/neovim#1135? It basically uploads a ZIP of everything installed by `make install`.
> I associated "nightly builds" with some unstable version end-users can execute on their machines, and this is only intended for API clients.

Agreed, but when we get a portable build it will work then, right? And then we won't have to change the URL.

> adding a note on the release page that this will only work on Travis

:+1: