Validate checks

[mariakimheinert]

In the future, we may want to add a warning if we get a check in the checks array that we cannot run.

Originally posted by @Michionlion in #91 (comment)

Currently, in gatorgrade/input/command_line_generator.py, we read in the data assuming that the user has provided a correctly formatted gatorgrade.yml file. If checks are improperly formatted (e.g. a shell check does not contain a command value), we allow them to fail at the "run" phase, which occurs in gatorgrade/output/output.py. We may want to add validation of checks at the "read" phase (i.e. in command_line_generator.py) that ensures that the gatorgrade.yml file contains all necessary data for each check and provides information to the user if it does not.