fix: Don't attempt to select initials when fullname contains non Latin characters

[kesara]

Fixes #759

[cabo]

Maybe this isn't important, as we are pivoting away from the use of initials, but I'm not sure that a Latin writing system is a necessary indicator for the use of basic initials heuristics. E.g., Cyrillic or Greek use the same initials heuristics (I think), while in general that is of course not true. So I think I'd like to have some documentation what exactly this change does.

[rjsparks]

I'm not sure where we would put such documentation - ultimately this is something the style guide will describe and may that's the documentation? From an implementation point of view, this is a rats-nest, and this fix is showing to get us what we need for the short term (an RFC containing a Japanese name is on the threshold of release). I suggest we approach this with incremental changes as real cases arrive.

[cabo]

Works for me.

There should be something that I can tell kramdown-rfc users about what these attributes mean and how they can be successfully used; this is what I meant with "documentation". That information would probably live in Github issues for quite a while.