[JeffDess]

Add aliases to mkTask so a task can be run under other names: lets colour runs color.

• Aliases work wherever lets accepts a task name: the dispatcher, -h, -s and completions (they also complete the task's options).
• lets --help lists them next to the task.
• An alias that clashes with a task name, another alias or a reserved name fails at load time.
• No flake app is created for an alias.

Closes #8