emoji 15.0

• Package version will now reflect the emoji version set by Unicode.
• Updated data to include Emoji v15.0.
• Added emoji_modifiers data set with information about emojis and their modifiers.
• Added emoji_modifier_extract() to extract modifiers from emojis. (#8)
• Added emoji_modifier_remove() to remove modifiers from emojis. (#7)
• Added zoo() to randomly sample animal emojis. (#10)