expose format and config through api

addresses #20

adds `jsfmt.getConfig()` and `jsfmt.format(js, [config])`

```javascript
var jsfmt = require("jsfmt");

var formatted = jsfmt.format(js, jsfmt.getConfig());
// the "default" config settings are loaded by default
// this is the same as above.
var formatted = jsfmt.format(js);
```