Allow the user to add/override parsers for a given file extension

[NextDesign1]

At the moment, the parser used for a given file extension is hard-coded. It would be great if this could be overridden in the configuration.

For instance, telling it that a new extension like .cxx should be parsed as C++, and the existing .c rule should be overridden to C++.

[MrDoe]

This can be useful. But right now I'm on vacation, so I'm planning to implement this in September.