Add "-T" option that shows actual build plan

[arcivanov]

Currently -t shows all available tasks in alphabetical order.

Using pyb -T [<task> [task]*] we should show an actual build plan as it would be executed without actually executing it.

The other alternative is to use --show-plan.

[arcivanov]

@mriehl review please

[esc]

I usually expect both a short and long option from my unix tools. I would suggest the following combinations:

• -t and --show-tasks
• -T and --show-plan

If --show-tasks is unlikely however, I would advocate for -T.

[esc]

--tasks and --plan or even -p instead of -T are also options.

[arcivanov]

@esc yep, I assumed there would be long names.