Run style50 only on student code

[brianyu28]

Use case is for CS50 Finance, where we only want to grade percentage of correct lines in application.py but not helpers.py. Right now the ECS script just grades the entire directory, and has no way of knowing that application.py should be graded but helpers.py shouldn't.

Think we might be able to let style50 (when a particular command-line flag is used) interpret the exclude files in cs50/checks to know which files to score?

cc @dmalan

[cmlsharp]

Where's the best place for it to look for the exclude file? Or should check50 just pass the location via command line option?

[dmalan]

Discussing tomorrow afternoon; will loop back!