Don't bind compass plugin twice

Status: **Ready for merge**

Reviewers: ** @jansepar  @noahadams **

## Changes
- In the compass plugin, check to see if it has already been setup, if it has blow that popsicle stand.

## How to test-drive this PR
- Run this new version of the mobify plugin on a project that is using 'compass_plugin' or 'compass_run' in its plugins
- When you run `mobify preview`, the message 'Binding Compass Preview' should only display once.

## Research resources
- (link1)