Add "exclude" feature to visualization

To reduce clutter and noise, I would love the ability to blackbox (either exclude or collapse into a single node) commits matching some criteria:
* all commits before a certain date or commit, or
* all unexplored commits (e.g. highlighted in blue below)

See also #40 (but that may not exhaustively solve this issue).

Inspired somewhat by [the blackboxing feature in Chrome Dev Tools](https://developer.chrome.com/devtools/docs/blackboxing#how-to-blackbox).

Review the existing visualization behavior and related issue #40 first, then compare the requested date or commit cutoff and unexplored-commit cases with the current design. Done means the visualization can exclude or collapse commits matching each stated criterion, reducing the clutter described in the issue.

Written by the indexing model from the issue text.