Fix multibyte handling

multibyte characters shouldn't be treated length with strlen. it should be used with strdisplaywidth.
For example, `あ` is 3 bytes in utf-8, but 2 cells width. So it should use strdisplaywidth().

For example:
![](http://go-gyazo.appspot.com/542ebd550a32547c.png)

This should be:
![](http://go-gyazo.appspot.com/7caee0dbca5735b6.png)

But currently:
![](http://go-gyazo.appspot.com/84dbf25cd542d535.png)

This change fix the issue.