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]