v2: offer easy way to upgrade ext_emconf.php version

t3xutils should automatically figure out the next version if I tell it that it's a bugfix, feature or major release.

Example:
```
$ t3xutils emconf:version bugfix
Version updated from 1.0.1 to 1.0.2

$ t3xutils emconf:version feature
Version updated from 1.0.1 to 1.1.0

$ t3xutils emconf:version major
Version updated from 1.0.1 to 2.0.0
```
