Ability to set vars on StackTraceFrame

[nicpottier]

Though we can't really get variable information at runtime, it would be useful to be able to set the vars attribute on StackTraceFrame so that we could annotate those frames. Specifically if you are using something like pkg/errors you may have annotated each level of your stack trace with a message, being able to see that message, which often contains useful information, with the frame in Sentry would be mighty useful.

Can prepare a PR for this if there's interest.