Pre- and post-processing steps

[zml2008]

Right now blossom just emits templates. I'd like to support some sort of system for post-processing to allow, for example, templates to be written in yaml and then converted to json on build, or to apply some sort of autoformatter.

I'm not sure what the most sensible way to handle this is, since the transformers would have to be provided on the worker classpath, and we'd want to pass options through to them.

Do we want a copyspec-type system so certain transformers only apply to certain template files?

These transformers should be able to transform file name and file content, i suppose. Does pebble itself become a transformer?