add optOccurence

[Kreijstal]

I was looking through the source code, and apparently there is no way to add a parser dynamically, since the function repeatable is not available :(

it'd be nice if you could add something like

masala-parser/src/lib/parsec/parser.js

Line 170 in 3fa77a2

return repeatable(this, l => l < occurrence, l => l === occurrence);

optoccurrence(occurrence) {
        return repeatable(this, l => l < occurrence, () => true);
    }

or even

occurrencerange(start,end) {
      return repeatable(this, l => l < end,  l => l >= start);
}

or both?

I would love to do a pull request, but I have no idea about haskell, and I can't understand what something like "// Parser 'a 'c => unit -> Parser (List 'a) 'c" means, so out of fear of just being wrong, I will just ask politely here 😁.

Yes, I know you could use something like .opt().occurrence(x) however, it seems wasteful, when x is big