Add `stacktrace_includes` option.

I have a Rails engine that makes database queries.
For people like me, it's nice to be able to tell Bullet
to include your own engine (or anything else you want)
in the stack trace.

Also fixed a bug where the "N+1 Query method call stack"
message was being shown multiple times if `call_stack_messages`
was called multiple times.
Merge pull request #146 from BMorearty/stacktrace-includes

Add `stacktrace_includes` option.