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.
Use message-signature-separator
Merge pull request #345 from DamienCassou/goto-begin-end-of-message

* better `M-<` and `M->` when composing a message.