Allow for customizing SASS number precision in Web Essentials Options

This adds the option to set the number precision for SASS.  the number precision will determine how many digits are output after the decimal place in any number in a Sass file.

![2014-06-20 13_46_12-greenshot](https://cloud.githubusercontent.com/assets/463685/3343755/7cbb8dfe-f8a3-11e3-9b22-d01d5d0cff4a.png)

---

Use Case:
====

Compiling [Bootstrap Sass](https://github.com/twbs/bootstrap-sass) requires Sass's number precision to be set to `10` instead of the default of `5`.  Allowing this change will let bootstrap (and other projects) output the values that their authors intended.

**SASS input**

    .example{ 
        line-height: 1.428571429; 
    }

**CSS output (with precision set to 5)**

    .example{ 
        line-height: 1.42857;  /* Boo! Number gets truncated! */
    }


**CSS output (with precision set to 10)**

    .example{ 
        line-height: 1.428571429; /* Yay, number is exactly what was written! */
    }
This one too :P