Split flx into 2 separate package

The flx repository actually contains two packages - `flx` and
`flx-ido` (which depends on `flx`). The `flx` is meant to be used by
various front-ends so it makes perfect sense to be distributed
separately from the `ido` front-end.
Merge pull request #889 from bbatsov/flx

Split flx into 2 separate package