Disable prototype methods when disabled

[EricMCornelius]

Currently the prototype additions to string don't seem to be disabled appropriately when not in a tty session. Following test case demonstrates:

ecornelius@ecornelius-T5-XE ~/Repositories/Posh % cat test.js 
#!/usr/bin/env node

var colors = require('colors');
console.log(colors.enabled);

console.log('test'.red);

ecornelius@ecornelius-T5-XE ~/Repositories/Posh % ./test.js 
true
test [red]

ecornelius@ecornelius-T5-XE ~/Repositories/Posh % ./test.js --no-color
false
test [red]