Enable custom profiling interval via CLI flag (#357)

[omarjatoi]

Issue: #357 (guest profiler support originally added in #301)

This PR adds the ability to pass a profiling interval to Viceroy, e.g. --profile=guest,/tmp/path,25 will set the profiling sample interval to 25 ms.

Open question(s):

1. It looks like wasmtime will parse the time including units, e.g. --profile=guest,/some/path,25ms. This PR assumes milliseconds by default. I can either add a small parse_duration helper function or use the humantime crate (used by wasmtime, so already an implicit project dependency) to stay consistent with wasmtime. Thoughts?
2. In Viceroy, the default sampling interval is set to be 0.05 ms (50 microseconds). The default interval in wasmtime appears to be 10 ms. Should Viceroy's default be changed?
3. Current CLI tests do not test anything related to profiling, I'm not sure how valuable those tests would be but I can certainly try to write something to cover the functionality/changes in this PR.