Add support for custom picker size

Picker size can now increase or decrease by using the width and height props:

By default the picker is 240px wide and ~315px tall (depending on the position of the categories panel). To change the width and height of the picker, simply:

<EmojiPicker width="315" height="280"/>
<EmojiPicker height="280"/> // width will default to 240
<EmojiPicker width="315"/> // height will default to 240

A word on height: The height you determine by the height property, is of the emoji-list only, the search and categories panel are added to the height you specify.