feat: calculate and display jitter (#39)

[fujiapple852]

Closes #39 (by @trkelly23)

The first of three PRs adding the 4 jitter columns to the backend and TUI + JSON:

Optional columns Jttr, Javg, Jmax & Jinta

1. The current jitter i.e. round-trip difference with the last round-trip ('Jttr')
2. The average jitter time for all probes at this hop ('Javg')
3. The best round-trip jitter time for all probes at this hop ('Jmax')
4. The smoothed jitter value for all probes at this hop ('Jint')

Testing by running with the optional jitter columns or specify JSON output.

cargo run -- -u --tui-custom-columns="THOLSRAVBWjgxi" freebsd.org

JSON output:

cargo run -- -u -m json freebsd.org