Pattern adapters for other patterning languages

[pcoccoli]

A way to implement #122 could be through "pattern adapters" - these would be modules that allow the user to specify patterns in something other than STIX patterns (or ECGPatterns). Some examples:

• Sigma: specify path to sigma rule file
• Yara: similar to Sigma (these would only be applicable to STIX artifact objects though)
• Snort: these might be short enough to specify inline (should probably only apply to network-traffic and maybe artifact objects, if they contain packet payloads)
• any number of graph pattern languages

[subbyte]

Good idea making the patterning sub-language extensible.

We may want to have an ABC for any adapter to implement two functions: to_stix() and to_firepit().