The completion engine maps completion locations to spans on a line, which
indicate whther to complete a command name, flag name, argument, and so on.
Initial implementation is simplistic, with some rough edges, since it relies
heavily on the parser's interpretation. For example
du -
if asking for completions, `-` is considered a positional argument by the
parser, but the user is likely looking for a flag. These scenarios will be
addressed in a series of progressive enhancements to the engine.
