Add a `ambiguousIsNarrow` option

[ehmicky]

Some users might want to override the default value for the string-width option ambiguousIsNarrow. See https://github.com/sindresorhus/string-width/issues/38 about reasons why they might want this.

Therefore, this library should allow passing that option.

Note: I can submit a PR if this sounds good.

[sindresorhus]

Now that it's fixed in string-width, do you still need this? Otherwise, I prefer to wait until someone actually needs this before exposing an option for it.

[ehmicky]

Exactly, this is not needed anymore, and I agree adding an option is not helpful anymore then. Thanks!