Provide a 'bump' mojo

The idea of the 'bump' mojo is that calling

```sh
mvn scijava:bump
```

in the top-level directory of parent POMs (such as [`pom-scijava`](https://github.com/scijava/pom-scijava) or [`pom-imagej`](https://github.com/imagej/pom-imagej)) will adjust all of the dependency versions defined in the `<properties>` section to the current release versions.

This job is currently performed by the [`bump-versions-in-pom.sh`](https://github.com/scijava/scijava-scripts/blob/4f2168a7a678b814771e50dfe17f10f5c2cfa6bf/bump-versions-in-pom.sh) script. The fact that this is a shell script starts to limit what we can do with it, though: it requires indentation with tabs to work, it has no idea about the XML layout of the `pom.xml` files, and therefore cannot use, say, XPath queries, and the shell script has no idea about Maven, of course, relying on a very stringent deployment process that is tightly bound to the ImageJ Maven repository that has been configured specifically to support the shell script.

Hence the drive to support the bump action as part of a proper Maven plugin.