Improve command line argument handling and help

- Add --duration and --sample_freq options
- Check range/primeness of --sample_freq option
- Added --flamegraph_file option
- Added #cfg[test] block for main.rs
- Add version locks in Cargo.toml
- Added docs/help for remaining arguments
- Add rstest crate; parameterize tests for sample-freq option
- Add doc/help for --filter-logs arg
- Use strings feature of clap crate to handle --duration option default properly

.gitignore

@@ -1,3 +1,3 @@
 /target
 flame.svg
-src/bpf/*_skel.rs
+src/bpf/*_skel.rs