Improve REPL directive support (#348)

* Improve REPL directive support

- use . on column 0 as directive prefix
- use `directives` object properties for genericity
- accept non ambiguous directive abbreviations
- reject invalid directive with extra characters
- simplify `handle_directive` and `handle_cmd`
- document ".help" instead of "\h"
- document ".load"

Author: chqrlie