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?

Let me know if there are any things in the code you would like me to fix.