Added warnings to command module

Generate warnings when users are shelling out to commands
rather than using modules

Can be turned off on a per-action line with the documented
warn=False flag. Can be turned off globally using
command_warnings = False in ansible config file.

Print out warnings using the standard playbook callbacks.

Created some additional tests in TestRunner.test_command
and also a demonstration playbook.
Slightly trim down the warnings list based on some things being pretty common
for local usage.
Docs for command warnings feature.