Add support for external FOP configuration files

[vjlamp]

Having a way of specifying an external FOP configuration files would be very useful.

Some tasks, such as registering fonts can only be achieved by configuring FOP. Since programmatic configuration is not a useful option for the fopRenderer plugin, using an external configuration is the way to go.

Hopefully this is something you would consider adding?

[ramonwirsch]

I'd consider accepting merge requests, but will not develop this myself.
See the 'Contributing' section I just added to the readme.