Add case for global exec

[tonybaloney]

Updates the general_bad_file_permissions plugin to give:

• Medium if group writable or executable
• High if global writable or executable

Closes #569

[lukehinds]

hey @tonybaloney

we also need to add the new stat values you added into the example file bandit/examples/os-chmod-py3.py

for example..

>> Issue: [B103:set_bad_file_permissions] Chmod setting a permissive mask 0o777 on file (/etc/hosts).
   Severity: High   Confidence: High
   Location: /home/luke/repos/bandit/examples/os-chmod-py3.py:14
   More Info: https://bandit.readthedocs.io/en/latest/plugins/b103_set_bad_file_permissions.html
14	os.chmod('/etc/hosts', 0o777)

[tonybaloney]

@lukehinds sorry I fell asleep on this one! Updated

[ericwb]

@tonybaloney I approved, wanna rebase and resolve the conflicts.