Align computation about how many chars were written to terminal
Align computation about how many chars were written to terminal

previously, we used runewidth.StringWidth() in the outer loop calling
printScreen(), but withing printScreen() we had code that expanded
a single character to up to 4 chars ("\t" == "    "), which cause our
drawing to go nanners after that point

This change make printScreen return the number of chars that were
written to the terminal, so we can use that value to advance the
drawing position in the calling routine

Fixes #226

ukitazume/peco,ken-saka/peco,polamjag/peco,peco/peco,dav009/peco