Fix alignment of titles/tags with multibyte characters in list index

The layout of the list index relies on calculating the length of the
title. Previously this assumed single byte characters, i.e no CJK
character; no mutli-byte UTF-8 characters; only plain "ASCII" characters.

It used the Python `len` function so `len("abc")` = 3, but also so does
`len("∞")`= 3.

Fortunately Vim provides a function for determining the display width of
a string.

Fixes #60