Performance optimizations (ca. Factor 75)

- Precomputed style function
- Skip arguments to array + join if there's only one argument (the
  common case)
- Merge multiple return statements to one

To calculate the performance benefit:

```javascript
var chalk = require('./index.js');
console.time('100000 iterations');
for (var i = 0; i < 100000; i++) {
	chalk.red('A string that is about 80 characters long (normal use I think?)');
}
console.timeEnd('100000 iterations');
```

Running this before this commit:
```shell
for i in {1..5}; do node time.js; done
100000 iterations: 19485ms
100000 iterations: 18933ms
100000 iterations: 19365ms
100000 iterations: 19332ms
100000 iterations: 18660ms
```

After:
```shell
100000 iterations: 268ms
100000 iterations: 261ms
100000 iterations: 264ms
100000 iterations: 259ms
100000 iterations: 254ms
```

Performance gain, taking the middle result of both:
```shell
19332 / 261 = 74.~
```

:star:
Sure, that's essentially how it was - but running some tests showed this to be slightly faster. Perhaps not worth the legibility tradeoff though.