Fix and improve rst_char_len

The previously used regex was plainly wrong (see #394). This solution should be more solid.

Tests with Python 3.4.1 (both manually defined `rst_char_len` and `vim.strwidth` yield the same output; only results shown):

```rst
****
jkfö
****

*************
란 무엇입니까
*************

**
我
**

Section name
============
```