Trace exception local variables and args per frame

New configuration options and defaults
config.locals.enabled = off
config.locals.max_trace_frames = 100

Turned off by default because this may not be performant just yet!

  1. Stores each Ruby process' local vars and method arguments
  2. When gathering exception data, "replay" over each frame to get the variables for that frame's context
  3. Dump the cache and repeat
If we assume the _typical_ singleton Rollbar use-case, Rollbar#start_tracking_locals _should_ only allow one active proc for `set_trace_func`.  Do we expect users to create multiple clones of Rollbar?