Schemdule

- Add `CallablePrompter`
- Fix bugs in prompter hubs
- Add `env` variable in schema
- Adjust extension framework

Add extension support.

- Redesign commandline user interface
- Add verbose & preview options
- Update schema
- Support custom and multiple prompters
- Support loading other schema
- Support `time` parameter

- Rewrite scheduler
- Add pending progress indicator

Core design and features.

- Python script for schema.
- Scheduling with Tkinter prompter.