Allow to tag or group TMs by categories

[unho]

For example Inkscape TM can be tagged as "graphics", and LibreOffice and Apache OpenOffice can be tagged as "office".

[jordimas]

I will be good to define how this should work. Some ideas:

1. A simple approach will be defining this kind of tags as meta information of the projects (in projects.json)
2. Use some kind of NLP system to detect the predominant topic of a translation using NLTK. We only need this on the source language (English) since the topic does not change with the translation.

I probably will start with 1) but definitely 2) can be interesting too to explore at some moment.

[unho]

@jordimas I was definitely thinking only on 1).