Add getConfig() method (#103)

[tonyganch]

Issue #103.

Examples:

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

[tonyganch]

Tests are added, branch is ready for review.