Plugin bootstrap proof of concept

Not really meant to be pulled, just an example how a plugin system could be bootstrapped:

* A `plugin` directory is created next to `lib`. JARs can be dropped in here.
* `jbake` script adds `plugin` to classpath (`jbake.bat` not modified and only tested on Ubuntu, not Mac).
* Plugins implement the `JBakePlugin` interface and declare themselves as a service for that interface.
* In `Main`, a `ServiceLoader` picks up all implementations of `JBakePlugin` and calls `init` on each one.