added attributes to grammar

This feature will permit one to use attributes that **raphael.js** provides, which will allow us to modify how text looks and behaves.

extended syntax can look like this:

```dot
Title: My Title [fill="red", font-weight="bold"]
Participant Andrew [fill="red", href="http://example.com"]
Andrew->China: Says Hello [fill="blue"]
Note right of China: China thinks\nabout it  [fill="blue"]
China-->Andrew: How are you?  [fill="blue"]
Andrew->>China: I am good thanks!  [fill="blue"]
```

![Image of output]
(https://rawgit.com/winfinit/images/master/js-sequence-diagram/issue28/output-example.svg)