Use issue title to define "size".

I am doing this in my project:

![screen shot 2013-12-30 at 7 37 17 pm](https://f.cloud.github.com/assets/193112/1821898/c9227048-7146-11e3-92b0-5b1b1fd81f9e.png)

And it accept float number as well. (`[1.5] Issue title`)

````json
{
    "use_title": true,
    "size_label": "\\[(\\d+\\.?\\d*)\\]"
}
```