More memory aware

[farzadghanei]

If the system is running out of memory, mute crashes and Go runtime dumps a stack trace. Would be nice to be able to improve on this. A couple of ideas:

• help to reduce memory usage (maybe guide Go runtime GC)
• Watch for low memory when accumulating the output, and dump whatever caught before the process crashes

[farzadghanei]

#20 restructured mute.Exec into 2 separate functions, which could help with GC being able to claim/reuse memory more efficiently, as some portion of memory would be out of scope sooner, before continuing with matching output against configured criteria.

As for watching for low memory, finding a platform solution would bring complications, and support from Go runtime to be notified of memory pressure is not as available. So let's wait to see what the runtime would provide in future.