Pre/post process callbacks/hooks

[nolimits4web]

This is also can be useful, if compiler will accept pre and post process hooks, it will allow to do extra modifications with input/output:

• preContent - hook that will receive input file string. It can return modified string that will be proceeded by compiler to ast and so on
• preAst - hook that will receive ast when it was just parsed to ast by component, but didn't go through any modifications/transformers yet
• postAst - hook that will receive ast after it passed all transformers and just before its compilation back to string in compiler
• postContent - hook that will receive compiled from ast script string, right before it will be saved to file. It can return modified string that will be saved to file

I think we may such hooks/callbacks on both root config level and separately for each compiler (vue, react)