Add CLI argument to override random chars for while/if statements

[MysteryBlokHed]

Right now, the tests for the contents of while loops have to use globs to find the correct files. If more than one while loop was to be used in a test, then it would take some extra code to find out which output file was generated by which loop. This problem gets more complicated with if/else statements (#84), since they generate more files and some files won't always exist.

Adding a CLI option to pass a list of chars to use instead (eg. --chars aaaa,bbbb) could solve this problem, since the output files will always have the same characters at the end.

I think that Clap can deal with multiple values passed for the same argument, but I'm not sure if it's comma-separated, or if you pass the argument more than once (eg. --chars aaaa --chars bbbb), or something else.