Add format argument to support wrapping of emojis

[smeijer]

Closes #44

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Relevant part of readme update:

format

emoji.emojify(str, onMissing, format);

As third argument, emojify takes an handler to wrap parsed emojis. Provide a function to place emojis in custom elements, and to apply your own styles:

var format = function (code, name) {
  return '<img alt="' + code + '" src="' + name + '.png" />';
});

var emojified = emoji.emojify('I :unknown_emoji: :star: :another_one:', null, format);
// emojified: I <img alt="❤️" src="heart.png" /> <img alt="☕️" src="coffee.png" />

[omnidan]

@smeijer thank you! looks good 👌

[omnidan]

@smeijer actually, I noticed an issue with this - when null is specified as onMissing and a format function is specified, even non-existant emoji are modified.

The question here is which behavior we want. I think it's probably better if the formatting only gets applied to existing emoji and the rest are left alone by default (when onMissing is not defined).

[omnidan]

also see #49

[omnidan]

published 1.7.0 with this merged

[smeijer]

Fixed the bug you've mentioned in PR #50