I'd like to contribute support for Vi/Vim modelines.  These are
somewhat similar to Emacs' local variables.  The basic idea is that
this makes it easier to collaborate with a software author who uses Vi
modelines to correctly set up their editor, and expects others to use
consistent settings.

This change should be covered by the copyright assignment my employer
(Google) already made for GNU Emacs.

This is my first Emacs patch, so I apologise for any misunderstandings
or omissions.  The patch itself is attached, but here is the ChangeLog
entry:

    Add support for Vi/Vim modelines.

    * lisp/emulation/vimvars.el: New file, implementing support for Vi
      modelines.
    * test/lisp/emulation/vimvars-tests.el: New file; tests for this
      feature.
    * doc/emacs/custom.texi(Vimvars): Describe this feature.
    * doc/misc/viper.texi(Modeline): Mention this feature.
    * etc/NEWS: Mention this change.