Allow overriding default processor configuration on a per-group basis.

[rlubke]

There may be cases where certain processors are very specific to a group that they deal with. In these cases, it would be nice to be able to override the default processor configuration and provide a specific configuration for a group.

This could also be a general optimization of the processor pipeline. Consider the case where one group does less processing, but another group is only interested in merging static content. The static content group has no need for less processing, so being able to assign the less processor to the groups that need it only provides a small runtime benefit (if you're using WRO dynamically vs at build time).

[alexo]

This is an interesting feature and it worth adding it. Thanks for reporting.