[RFC] Build reports independently and in parallel.

#12 actually didn't comprise that many changes, so I went ahead and implemented it to see if/how it works in reality.

You can see the result at https://github.com/fwalch/doc2/commits/gh-pages (only commits from Aug 20 are relevant). This is the output of two builds, one is at https://travis-ci.org/fwalch/bot-ci/builds/33041843. Note that there is no commit for `doc-index` (pull index.html from neovim.org/doc_index), because that file didn't change at all.

The only thing I'm not too happy about is the very last line of publish-reports.sh:

```bash
until (git pull --rebase origin ${DOC_BRANCH} && git push https://${GH_TOKEN}@github.com/${DOC_REPO} ${DOC_BRANCH}); do echo "Pushing to ${DOC_REPO}."; sleep 1; done
```

Force-pushing will push the full local history, not just the last commit, so I think there is no other way than to update the local clone before pushing. I hope that this is an acceptable "hack", however.
If I only change this here, there will be both `build-reports` and `reports` directories in `neovim/doc`. Created neovim/doc#1 to take care of that.