Add option to specify own preproccessor

[J0J0HA]

An option to specify a custom file preprocessor would be really cool. There is already the option to toggle a preprocessor for replacing *-imports, but we (GSG-Robots) have built our own preprocessor (https://github.com/GSG-Robots/comPYner/) to support all types of imports, because we want to split up our code into multiple files.

I think it could be implemented like this: You can specify a command that is being executed. The file contents are passed into stdin and the result is read from stdout and then uploaded to the brick.

What do you think about implementing such a feature?