NarProcessLibraries enhanced command construction

Updated the nar-process-libraries mojo to allow more flexibility when
constructing the command line to be executed.  The configuration now includes a 'token' which is substituted, within the plugin, for the full path of the library being operated on, allowing use in command lines that require the target to be somewhere other than the end of the line.
As a side effect, the configuration required has been updated and documented
accordingly.  An example is given below:
```
<processLibraries>
    <token/>
    <commands>
        <command>
            <libraryType/>
            <executable/>
            <arguments>
                <argument/>
            </arguments>
        </command>
    </commands>
</processLibraries>
```