Allow for customizing SASS number precision in Web Essentials Options

I would love to have this added. The changes proposed here seem to work from my manual testing, but this is the first VS Extension I've ever worked in before.

![2014-06-20 13_46_12-greenshot](https://cloud.githubusercontent.com/assets/463685/3343755/7cbb8dfe-f8a3-11e3-9b22-d01d5d0cff4a.png)

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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! */
    }