Add option to sort fully matched candidates first.

[okamsn]

Hello,

This is a first pass at sorting fully matched candidates before others. I asked about this in #75.

Do you see places where this could be improved? I do not have much experience with making Emacs fast.

Thank you.

[raxod502]

Thanks! I made a few changes. Mostly refactoring for clarity and avoiding code duplication, plus I called length before comparing the strings, which should presumably be a minuscule amount faster.