feat(task): Add method exists.

return try is already are registered.
Merge remote-tracking branch 'grunt/master'
test(task): run task b, require task a with arguments.

Task b has to succeed if a:x ran first and succeed.

Related issue #998.
WIP: Check name in object[name:target].

require research.
WIP: Expand success parameters.

This it’s used in the require function.
WIP: Run task b that require a, after run a:param.