load subcommands lazily
load subcommands lazily

This makes some subcommands (and actions like --help and --version) execute
much faster. This has the side-effect of making tab completion much faster too.

iriark01/yotta,ntoll/yotta,autopulated/yotta,BlackstoneEngineering/yotta,autopulated/yotta,ARMmbed/yotta,ARMmbed/yotta,stevenewey/yotta,stevenewey/yotta,ntoll/yotta,eyeye/yotta,BlackstoneEngineering/yotta,theotherjimmy/yotta,iriark01/yotta,theotherjimmy/yotta,eyeye/yotta