Add White frowning face (megafrown)

[axelson]

http://emojipedia.org/white-frowning-face/ and http://www.unicode.org/emoji/charts/full-emoji-list.html#2639

I'm not sure what a good name for the emoji is. I choose megafrown because that is listed as the alternate. Perhaps "frowning_face", "white_frowning_face", or maybe just "frown" would be better?

[mwunsch]

I'd prefer not to choose a name arbitrarily. Consult http://emoji.codes

(They use :frowning2:)