Add way to match prefixes, similar to `partial-completion`.

[okamsn]

This adds a filtering regexp similar to the partial-completion style provided by Emacs.

This is convenient when quickly editing history items to find something similarly named, or just when being used to this particular behavior in default Emacs.

For example, t-t-l matches toggle-truncate-lines (though I prefer using the initialism style in this case), and fi--a-po matches find-file-at-point, find-function-at-point, and other similarly named symbols.

What are your thoughts?

[okamsn]

I have tried to make the suggested changes, and to mention the prefix filtering in the proper places.

Is there anything else you would like changed?