[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 (ref #12).
> Seems like we could add BUILD_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)" here.

The problem with `BUILD_DIR` is that it's used when including the "library" `.sh` files, e.g.:

```bash
source ${BUILD_DIR}/ci/common/documentation.sh
```

If it's done that way, `BUILD_DIR` can't be defined in `documentation.sh`. I wanted to have an absolut path for the includes, otherwise only `./ci/doc-index.sh` and not e.g. `cd ci && ./doc-index.sh` would work to call these scripts.

> Later we might want a common/init.sh and move some things from documentation.sh into there.

:+1: I wanted to do that when there are other build types (coverity, deps) and it's more obvious what's common between all of them.