Add platform-specific tag to rules

[r0binak]

This is only necessary for use in certain environments.

[tstromberg]

Spoke to @r0binak via e-mail, the gist here is to provide a way for folks who are only interested in rules that impact a single platform to filter by or ignore other platforms. For example, for a Linux CI/CD platform, you probably don't care about Windows hits.

What I propose is adding GOOS-style tags to our rules that only impact a subset of platforms:

• linux
• darwin
• windows
• solaris
• ...

For 3rd party rules, we'll need to apply some heuristics to auto-tag rules: for example, rules that contain Win32 in their name should be tagged as Windows.

[tstromberg]

This has launched for a fair number of 1st party rules - we still need to add code to automatically do this for 3rd party rules based on heuristics (rule name, description)