add override option to add_translation_dirs

[mmerickel]

I debated with myself for a very long time whether to set override=True by default or not and in the end I simply cannot come up with a compelling reason to break bw-compat. I took a page out of pyramid_jinja2's book with add_jinja2_search_path which also adds new items with lower priority than previously added items. This is consistent with that, as well as previous behaviors.

fixes #1474

[piotr-dobrogost]

add_jinja2_search_path which also adds new items with lower priority than previously added items

Heh, then it's the wrong default there, too :)

I simply cannot come up with a compelling reason to break bw-compat.

The fact no one before @rbu raised this means that those, who use add_translation_dirs use it only to add translations and not to overwrite them. This means they have no overwrites intended to work because if they had got any they wouldn't have worked and they would have submitted issue. Conclusion; if overwrite=True breaks something it means it was already broken (by having needless overwrites not meant to overwrite).