Only Latin alphabet

The list of possible initial letters is hardcoded in line 21:
```
var letters = ['_', 'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z', '-'];
```
What if the list is entirely or partly in Hebrew, for example?

Why not form `letters` dynamically as in my pull request?