[RFC] Translation report & consistent report formatting

Should probably have waited for neovim/neovim.github.io#48, but I wanted to get started with this to get feedback.

* Output repo [`fwalch/doc`](https://github.com/fwalch/doc)
* Doxygen output: https://fwalch.github.io/doc/dev
* Clang report (from #2): https://fwalch.github.io/doc/build-reports/clang
* Translation report: https://fwalch.github.io/doc/build-reports/translations

@Stefan991, @justinmk: As far as I remember, Doxygen has support for "custom pages". Would it make sense to integrate the translation report there (it's kind of code-related)?

This uses the CMake translation targets from neovim/neovim#928.
@jszakmeister, I use `make cmake` to get to the `update-po-*` targets, but this downloads all external dependencies.. is there a way to circumvent this, since it's not really needed in this case?

Ping @elmart
I think you can avoid the `export` and `unset` steps by assigning the vars on the same line as the command call, like this:

```sh
report_title="${1}" \
report_body="${2}" \
report_output="${3}" \
... \
envsubst \
    < ${BUILD_DIR}/templates/report.sh.html \
    > ${report_output}
```