[WIP/RFC] Translation report

Should probably have waited for neovim/neovim.github.io#48, but I wanted to get started with this to get feedback.

The [build](https://travis-ci.org/fwalch/bot-ci/builds/29689975) results have been pushed to [fwalch/docs](https://github.com/fwalch/docs).
You can have a look at the beatiful translation report at https://fwalch.github.io/docs/build-reports/translations.html
The doxygen build also seems to have worked fine: https://fwalch.github.io/docs/dev/

@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
Change this to `OLD_PO_FILE_INPUT=yes msgfmt -v ...` and you'll get rid of the warnings about ja.sjis's invalid code sequences.