Add file watching mode

[cassaundra]

Resolves #10.

Implementation

Usage

Activated with -W/--watch command-line arguments.

Proposed watch mode implementation:

1. Evaluate the build tree as if in normal run mode.
2. Initialize file watchers (with notify) on all files according to task sources.
3. When a file changes, find all tasks that use that file as a source, and re-run all in breadth-first order.

Notes

• File watch updates should be debounced
  • Needed for batch operations (e.g. git-checkout) and certain editor save file implementations which produce multiple updates for a single file.
  • notify provides notify-debouncer-mini and notify-debouncer-full for this purpose.
• Interruptible tasks
  • It's convenient to interrupt tasks to save time and restart services (e.g. npm serve).
  • Implement by setting EngineState::ctrlc.
  • In case some cases tasks are sensitive to interruption. Could be resolved with a cleanup block and/or --no-interrupt flag added to tasks.