add global options, make -h a proper option

[jonsequitur]

This reimplements help as a proper Option, which fixes #691. In order for the help option to be a single instance, looked up using FindResultFor after parsing, I also added support for global options (#587).

This should also fix #766, the root cause of which is #691.

This will also fix #506.

[Keboo]

This makes the assumption that .UseDefaults() includes help. Though probably reasonable, I think it might be more clear to only test with .UseHelp()

[Keboo]

Same comment here. Consider use .UseHelp() rather than .UseDefaults()

[Keboo]

Consider .OfType<Command>() on the foreach to reduce nesting.

[jonsequitur]

A consequence of this change is that it also adds help output to --help which is a fairly common behavior and has been requested (#506), but it might be a little too helpful.

Usage:
  myapp [options]

Options:
  --version                Show version information
  -h, /h, -?, /?, --help   Show help and usage information
  -f                       Do some other stuff

[Keboo]

Any thoughts on changing the output on help to not display items that only differ by prefix? I would assume that would then make the output look something like:

Usage:
  myapp [options]

Options:
  --version               Show version information
  -h, -?, --help          Show help and usage information
  -f                      Do some other stuff

[jonsequitur]

I like that. It does leave aside the question of Windows-style option prefixes (#351).

[KathleenDollard]

1. Looking at the issues list, a trifecta is always nice :)
2. I think this is a good change. I do think it highlights that we have more work to do on the help output (including prefixes and how --help appears from this conversation). I don't think that should hold up this PR.

Let's do it!

[jonsequitur]

Any thoughts on changing the output on help to not display items that only differ by prefix? I would assume that would then make the output look something like:

Usage:
  myapp [options]

Options:
  --version               Show version information
  -h, -?, --help          Show help and usage information
  -f                      Do some other stuff

@Keboo Looks what you've made me do...

c9d2900#diff-ed0644cdb2d64176756014bb5d1a7115R335-R342

[Keboo]

@jonsequitur nice I like it