CLI settings for dlang-tour

[wilzbach]

The idea is to give people a convenient way to run the Tour with just their language, s.t. the translators can work directly in their repo without needing to deal with committing in git submodules.
If only a specific language file is selected, this should also be the default language for the web interface.

Variant 1

dlang-tour --lang-de .

(A bit more complicated as args need to be parsed manually)

Variant 2

dlang-tour --lang de --lang-dir .

Or other ideas. I personally like variant 1.

(part of #454)