This relies on the new `read --line/-L` support as an entire parser for
the output of `./configure --help` was written in fishscript. Also
doesn't work without 72f32e6d8a7905b064680ec4b578c41dea62bf84.
The completion script is slow... a function of both the autotools
configure script itself being written in a shell script combined with a
fishscript output parser.
fish's own `./configure --help` takes around 350ms to execute, while
`__fish_parse_configure ./configure` (which runs that behind the scenes)
takes around 660ms to run, all-in-all - a not insignificant overhead.
Output can be cached (based off of ./configure hash or mtime) in the
future if this is a big deal.
