make ambiguous east asian character width configurable

Trying to make ambiguous character width in [East Asian Width](http://www.unicode.org/Public/UNIDATA/EastAsianWidth.txt) configurable.

TODO
----------
- [x] Make configurable
- [x] Update tests
- [x] Think about reasonable default again
- [x] README


ref:
- http://www.unicode.org/Public/UNIDATA/EastAsianWidth.txt
- https://www.npmjs.org/package/eastasianwidth