Support for shorthand emoji input

[Tor Kringeland]

Currently, inputting some common emoji using `C-x 8 e e' is quite
cumbersome.  /E.g./ to input `👍' one has to write

  C-x 8 e e b h n a a

How about having a subcommand (/e.g./ `C-x 8 e c') and allow users to
specify a custom list of emoji?  It would work similar to `C-x 8 e r'
except that the options would be constant.  Personally I don't look at
the transient menus when inserting my common emoji, as it's part of
muscle memory, so this would serve a distinct purpose from `C-x 8 e r'.