Improve completion behavior, auto choose simple or flex style (#454)

[lorniu]

The default sly-flex-completions is very powerful, but most of the time it gives too many, and can not put the most wanted first. (#454)

Therefore I give this solution. By default, the sly-simple-completions works, if you want to switch to sly-flex-completions just type any dot (.), ie, make sure any dot in the completing pattern.

This allows for a flexible switch between the two.