Add a command line interface

from issue  #14 

### API
`specificity-graph style.css`
`specificity-graph style.css -o my-output-directory`
`cat style.css | specificity-graph > specificity.json`

It creates a report in html (data in a JSON).

#### Ex:

`$ specificity-graph style.css css-specificity`

```shell
.
└── css-specificity
   ├── index.html // Graph
   ├── specificity.json // Stats for your css
   └── dist
       └── bundle.js // JS for the graph
```