Allow matching in middle of string

[duetosymmetry]

This only requires changing startsWith to includes. However, not everybody will want this behavior, so best to have it as an option. More generally, could take a binary comparison function cmp(a,b) which will take as values a=_option.get().textContent and b=event.target.value. The default comparison function would be a.toLowerCase().startsWith(b.toLowerCase()), mid-string matching would be with includes, case-sensitive would omit toLowerCase, etc.