Small improvements and bug fixes

Improvements:

• New Editor constructor. You can use it to focus, get or set editor value, like:

var Twemoji = new TwemojiInput(document.querySelector('textarea'));

Twemoji.Editor.value('Custom value ✌️');

• 2x smiley icon. So it looks better on screens with big PPI, like retina.

Bug fixes:

• Emojis now placed at correct caret position
• Fixed custom scroll bugs