Added bower version command, like npm version

I'm a happy user of [`npm version`](https://npmjs.org/doc/cli/npm-version.html), and I needed the same functionality in bower, so I decided to write the functionality (inspired by npm's implementation).

It works just like `npm version`:

```
version [<newversion> | major | minor | patch]
```

I didn't find any tests for the other bower commands, so I'm in doubt of how to write automated tests for this command? :whale2:

Let me know if there are any things in the code you would like me to fix.