more extension methods

[samuelcolvin]

extra extensions:

• on_save modifiers which run before files are saved
• override_copy which modify the copy command both in render and copy_assets
• override_webpack to modify webpack args/env
• modify_data to change/extend data
• sass functions

[samuelcolvin]

override_copy should allow stuff like not always renaming files and copying some files to other locations, eg. favicon.ico