Transformed the help management to be implemented not using a command (CRAFT-542).

[facundobatista]

This has several benefits:

• The help handling is now more straightforward

• The COMMAND_GROUPS constant does not include anything related to help anymore (more app isolation ot a future lib)

• We're able to provide better messages when help was "requested wrong" (e.g. with too many parameters or an incorrect command); IMO this is very important, as we're able to guide the user better in a situation when they're a little lost (there is a reason they are asking for help...)

• All help handling happens on the "initial args pre-parsing", and not one part there and one part when the command is run (this is transparent for the user, but useful for when Dispatcher will be in other lib)

We have only one minor drawbacks: 'help' itself, as a command, is not listed in the "basic commands group" anymore.

Note I added a ProvideHelpException exception to interrupt the normal flow of execution and finish the process providing a guiding text when help was requested correctly. This was a simple way to achieve this branch targets without moving a lot of code around, it may change in the future to be more simple when we isolate the Dispatcher further.

[sergiusens]

Looks good, I do find it a bit confusing as the variable names don't follow the convention for commands, options and arguments seen in other CLI implementations and their conventions. A varname refactor could perhaps help.