Plugin params for tests & removeAttrs plugin

Hey

I've added a removeAttrs plugin that basically can be configured to filter out attributes that match a certain regexp pattern.

I use this basically to remove "fill" & "stroke" attributes so the can be styled with css. However I thought it make sense to give an option to filter out any particular attribute.

By this I took the chance to refactor the way plugins are tested a bit and added the option to specify plugin parameters in JSON form. This would help to test the plugins with various configurations.

Many thanks!
Ben