Add 'type' option for multifiles

[tjmoore]

Currently you can specify 'type' option for the preprocess action, but as far as I can tell you can't specify this individually for each file in a multifile declaration.

It may be useful to be able to specify the type per file. Obviously a workaround is a different action for each type and only include the files for that type, although the action declarations could increase in number dramatically if there are many environments being configured.

[BendingBender]

Could you provide an example config that illustrates how you think this feature should look like?