Add a "--pull" option to the switch command

[amateescu]

Fixes #47.

• ddev switch --pull <project> <branch> fetches the branch and fast-forwards it
  before Composer runs. Works for core and for contrib modules.
• Only a fast-forward. A diverged branch or a dirty working tree is left alone
  with a note, and the Composer step still runs.
• ddev mr shares the same fast-forward helper: quiet on success, and no longer
  reports a divergence when local changes were the real cause.
• The host commands show their usage instead of ignoring extra arguments.
  ddev switch core 11.x --pull used to drop the flag without a word.