Yargs: Add `-h` short option

[alexreg]

Could we have an -h short option, equivalent to --help?

[Morishiri]

@bcoe what do you think about that?
What about programs which are using h as alias for some command already? Will it break them?

[alexreg]

It could be configurable, of course. :-)

[bcoe]

@alexreg @Morishiri I thought about this when I made --help default to enabled. my main concern is that I think it's too likely that -h will collide with people's existing configurations (--help and `--version are specific enough that I thought it was safe).

You can totally just add .alias('h', 'help') to your configuration though, to get the effect.

[alexreg]

Ah right, that should do the job then. Thanks!