Support subcommands when querying for help

[natecook1000]

Currently, running swift help package (when the swift-help from this package is installed) prints the help screen for the swift package command. However, subcommands and sub-subcommands are not accessible:

$ swift help package clean
Error: Unexpected argument 'clean'
Usage: swift-help [<topic>] [-show-hidden]
  See 'swift-help --help' for more information.

This adds an array that captures additional subcommands and passes them along to the matched command:

$ swift help package clean
OVERVIEW: Delete build artifacts

USAGE: swift package clean [<options>] --enable-index-store

OPTIONS:
  -Xcc <Xcc>              Pass flag through to all C compiler invocations 
  -Xswiftc <Xswiftc>      Pass flag through to all Swift compiler invocations 
  -Xlinker <Xlinker>      Pass flag through to all linker invocations 
  ....

[owenv]

This looks good to me, thanks!

[natecook1000]

@swift-ci Please test