Todos:

Inject context environment variables into shell interaction. (File, selection_start, selection_end, prev_shell_command, if running script)
Improve classic.rs to support all of ed's command line arguments
Implement missing features from GNU Ed.
- List more missing features in README.md (look into GNU Ed manual and compare to add-ed).
- Implement missing features and remove them from listing in README.md.
  - 'g' command should accept an argument for case insensitive matching.

Look over undo/redo

Add absolute indexing to the 'u' command (u*0 = go to index 0 in history)
Possibly eventually add reverse/forward snapshot label search (u?^e? would search backwards to the last previous 'e' command (just regex))
Possibly a way to clear the history (probably as a subcommand/argument under 'U', perhaps better as a distinct command)
Some flags to print history in different ways (the 'U' command).
- 'a' to print absolute indices for the snapshots
- 'A' to print the whole history
- integer to give a specific snapshot to print nearby snapshots to
- '$' to print snapshots relative to the last existing snapshot

Make macros more useful by:

Adding some per-macro configurations, such as:
- Abort on error, abort on error except NoMatch and NoOp, or ignore errors
- Modifies buffer or not (if not we auto undo to before execution and delete its potential future, ie. make modifications not have an impact)
- Snapshot for each command or for the whole macro