Feature/context notation

Mostly in the backend for now https://github.com/texttochange/vusion-backend/pull/43

Need input on how strong we should run the cakephp validation? 
- only check the domain ```[context.xxx.xxx.xxx]```
- check the special case of ```message``` such as ```[context.message.<number>]``` and ```[context.message.<after|before>.<number>]```