Allow blacklisting a file from slicker

[benjaminjkraft]

Sometimes you have a file you know slicker is not going to do a good job of. (For example, lib/pickle_util.py in Khan/webapp.) It would be great if you can just blacklist those -- and instead of doing anything Slicker will read them, see if it would do anything, and then just print a warning saying so. That way you know to modify it, but we don't do something wrong.

This requires a way to config those. One option is a # slickerignore comment. In principle we could even apply it to a range of lines.