Better `M-<` and `M->` when composing a message.

This commit provides the following 2 functions that help the user go to
interesting positions at the beginning and end of the message being
composed. These 2 functions are bound to the standard `M-<` and `M->`
keys (or more precisely, to the keys where 'beginning-of-buffer and
'end-of-buffer are bound).

mu4e-compose-goto-top: Go to the beginning of the message or buffer.
  Go to the beginning of the message or, if already there, go to the
  beginning of the buffer.

mu4e-compose-goto-bottom Go to the end of the message or buffer.
  Go to the end of the message (before signature) or, if already there,
  go to the end of the buffer.