Add getConfig() method (#103)

Issue #103.

Examples:
```js
var Comb = require('csscomb');
var c = new Comb();

// Pass no arguments:
c.getConfig(); // default `csscomb.json` object is returned

// Pass a valid name:
c.getConfig('csscomb'); // `csscomb.json` object is returned
c.getConfig('zen'); // `zen.json` object is returned
c.getConfig('yandex'); // `yandex.json` object is returned

// Pass something invalid:
c.getConfig('nani'); // throw Error
c.getConfig(16); // throw Error
```


![tumblr_mhxs03dwvw1s4nb6vo1_250](https://f.cloud.github.com/assets/872004/1592404/e0574fcc-52ba-11e3-975f-8ddb440759b6.gif)