Add --help support for subcommands

[jnowjack-lucidchart]

Fixes #174

Not sure if this would be the best way to support the feature, but it was my attempt to do so.

./mill example.hello2 --help output is left unchanged:

$ ./mill example.hello2 --help
[108/108] example.hello2.run
[108] Available subcommands:
[108]
[108]   foo
[108]     --bool          Example flag
[108]     -f --foo <str>  String to print repeatedly
[108]     --my-num <int>  How many times to print string
[108]
[108]   bar
[108]     -i <int>
[108]     -s <str>        Pass in a custom `s` to override it
[108]

./mill example.hello2 foo --help prints out the help message for foo:

$ ./mill example.hello2 foo --help
[108/108] example.hello2.run
[108] foo
[108]   --bool          Example flag
[108]   -f --foo <str>  String to print repeatedly
[108]   --my-num <int>  How many times to print string
[108]