refactor using Keyword and Emoji

[pietroppeter]

type
  Keyword = distinct string # e.g "grinning"
  Emoji = distinct string # e.g "😀" 

I think this could be very useful, in particular to add a match functionality that could be used to refactor emojize and will give an easy implementation of #4 (and possibly also #7).

type
  Captures[T] = seq[tuple[index: Natural, pattern: T]]

func match(text: string): Captures[Keyword]
func match(text: string): Captures[Emoji]

Also one could support an Emoji by UniqueKeyword (list of UniqueKeywords generated when parsing emoji data) and Emojis by Keyword.