Automatic help text generation about yazap

[prajwalch]

Hey,
It will be definitely implemented 😁 but after v0.1.0 release. The only thing left to implement for v0.1.0 is arg validation and logging errors plus updating/improving README docs and example after that i will start to implement generating help and others.

[prajwalch]

Hi, thank you for creating this awesome library.

I have one feature request. (it could have already been considered though)
It would be more helpful for users to have automatic help text generation based on configured flags.

Yazap already has a Command.newWithHelpTxt function to set the about field, but currently, it doesn't have any way to output its information except for directly reading about and constructing our own help message by ourselves.

Now auto help message is implemented on a4df0eb with some breaking changes. I forget to mention this issue on commit therefore leaving a comment here.