[mixpanel] Company Attributes

[SegmentDestinationsBot]

Migrated from #30 by @jkindness

Since mixpanel does not accept object properties, wouldn't it make sense to check if "company" is being passed in as an object and if it is, flatten it. In other words, since segment allows you to specify a company object in the identify call, this code could simply take the object and translate it to "company_name", "company_plan", etc.