Improved printing and RME checks

`StyleRubric`'s `error_tracker` is now a dictionary that holds lists of errors for each file. This allows for easy printing of errors by file (#65).

`StyleRubric` now also keeps track of names of functions that have RMEs. This resolves the bug that caused an error for function implementations, even when their prototypes above `main` had an RME. Later, we'll have to improve this by keeping track of functions have RMEs for each file; then look for RMEs only in files that are #included.

Finally, I added a check for the minimum number of lines of comments in a file. The current ratio is pretty liberal, 1 line of comment for every ten lines. This ratio and the maximum line length are set in `rubric.ini`.