SubCommand inherit ErrorHandling from parent

The default for `ErrorHandling` was changed from `ContinueOnError` to
`PanicOnError` 8c20e9b. However this didn't apply to sub-commands, which
continued to use `ContinueOnError` and would execute the sub-command
function (in addition to printing the usage message) when not passed
mandatory arguments.

Fix this by copying `ErrorHandling` from the parent when adding a new
sub-command. The additional test asserts that the sub-command raises a panic
and doesn't execute it's contents when it doesn't receive the correct number
of arguments. Mute is required to prevent the help message being printed
during the tests.
Correct name of describe block for AddSubCommands

This block tests AddSubCommands rather than AddSubCommand.
Merge pull request #10 from dcarley/subcmd_errorhandling

SubCommand inherit ErrorHandling from parent