Support for RET in loccur-mode

Hello Alexey,

I find loccur to be an awesome addition to emacs and it is now a big part of my daily use. However, the fact that "RET" was inserting a newline in loccur-mode instead of calling `loccur-current` was annoying me so I hacked your code a bit to have this feature.

To achieve this, I had to modify several function but the core ones (those dealing with overlays) are untouched --- their understanding is way beyond my skills anyway.

Highlighting the matching regex's was also sometimes useless for me so I added the possibility to desactivate it. The default behaviour is not modified though.

I also updated the README file to provide examples of the power of your program!

I hope my modifications are useful; at least they are for me :smile: Anyway, I thank you again for the great work.

Cheers,

Léo