* Avoid redundant string splits
There likely isn't actually more than once to split in the source
strings in these cases, but avoid doing so anyway as we're only
interested in the first.
* Avoid redundant completion output target evaluations
The target is not to be changed while outputting completions, so resolve
it only once.
* Avoid redundant active help enablement evaluations
The enablement state is not to be changed during completion output, so
evaluate it only once.
* Preallocate some slices and maps with known size
* Avoid some unnecessary looping
* Use strings.Builder to construct suggestions
Fix some typos in README and comments.
Move arg validation to after flag validation so that the help flag is run first.
Pass the same args to ValidateArgs as the Run methods receive.
Update README.
Signed-off-by: Daniel Nephin <dnephin@gmail.com>
