[RFC] Refactor into standalone build scripts.

A build is [here](https://travis-ci.org/fwalch/bot-ci/builds/33169716), output (from multiple builds) in [fwalch/doc2](https://github.com/fwalch/doc2/commits/gh-pages).

From the updated readme:

> To generate individual reports locally, execute `./ci/<build script>`, where `build script` is any executable shell script. Override environment variables as necessary, e.g. `NEOVIM_REPO=<username>/neovim NEOVIM_BRANCH=my-branch ./ci/clang-report.sh`.

I think this also improves high-level readability of report generation quite a bit:

```bash
# clang report
DOC_SUBTREE="/reports/clang/"
install_dependencies
clone_doc
clone_neovim
generate_clang_report
commit_doc
```

Next step is to install only required dependencies for each report.
Why all these variables are placed in `ci/common/documentation.sh`? I don't see existing better place though. Perhaps it is worth to add `ci/common/environment.sh`?