Add support to track module stealth in metadata

[kpcyrd]

This allows adding:

-- Stealth: <value>

to the modules metadata to document how noisy the module is.

Valid values:

• passive: No traffic is sent directly to the target. This includes http requests, but also dns traffic and syn packets. Running a passive module should be undetectable by the target. I'm not sure if social media modules (-- Source: accounts) should be considered passive.
• offline: The network is not used at all. Definitely undetectable by the target. A module like this would probably request, process and update data that was added by other modules.
• loud: The module is very aggressive and likely to cause alerts. This may involve bruteforcing directories with http requests or subdomains, or anything that may cause emails to be sent (for example password resets).
• normal: The default if nothing is specified. You'd only use this for something like pkg list --stealth normal.

This still needs proper documentation for readthedocs.

It should also be possible to do:

sn0int --stealth passive

which would prevent running any modules that aren't offline or passive. A user would need to overwrite this with run --stealth normal.

Related to #170.