[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`.
This `else` branch will be taken if the release already exists (so for every build run after the very first one). Instead of re-creating the release, the script will just delete the release assets (i.e. nightly build tarball from previous build) and continue with updating/uploading at line 70.

The `|| exit` at the end is only triggered if there's an error (i.e. non-zero return code) calling the GH API. `bash -e` terminates if there's an error in a called program, but not if it there's an error in a subshell, so this explicitly checks for a non-zero return value and exits if there is one.