Add MISP Export

[MarkDavidson]

Tram is super useful and could cover the gap between unstructured information and structured information in a TIP like MISP. Thanks for the work! Looking at the code, if we extend WebAPI to add a new export function, I suppose that's the best way to do a MISP export to push the result in a MISP instance. We might extend the MISP objects to add the information contained in tram. Let me know if this make sense and we will start to work on it.

Originally reported by @adulau in mitre-attack/tram#2