Endpoint for adding/training a word

Currently, there's no endpoint for `varnamc -s ml --train PATTERN=WORD`. Adding that would be nice.

Example usage :
```
varnamc -s ml --train chrome=ക്രോം
```
Endpoint :
```
POST /train
lang=ml
pattern=chrome
word=ക്രോം
```

#17 

cc @athul @joicemjoseph