Remove the "length" or "offset" parameter from command completion methods

[TheFriendlyCoder]

Currently the Friendly Shell APIs force completion methods to have 3 mandatory parameters: a list of parsed parameters, and index number for the parameter to be populated, and a length/offset value within the token where the cursor was located. This model was a reflection of other APIs like the built in cmd class, but honestly I can't see a good reason to keep it in our APIs here. I propose removing this seemingly superfluous parameter to simplify the calling code.