Abort with usage help on wrong number of CLI arguments

- rbenv-install accepts 0 or 1 argument
- rbenv-uninstall accepts 1 argument
- ruby-build accepts 2 arguments
Improve ruby-build test for too many arguments

Use exactly the same arguments that would have `ruby-build` succeed at
executing, but add one more extra to test argument counting.
Perform CLI argument validation as early as possible
Provide CLI usage help on stout/stderr appropriately

When requested via `-h|--help`, usage text should be displayed on stdout
and the exit status should be 0.

When usage text is shown due to invalid arguments, it should be
printed to stderr and exit status should be 1.
Extract ruby-build usage from comments like we do in rbenv
Capitalize "Usage:" header for consistency with rbenv help system
Move rbenv-install/uninstall tests to `rbenv.bats`
Have rbenv-uninstall respect `-h|--help`
Merge branch 'extra-arguments'

Fixes #667, closes #668