Hi Joao and others!

The LSP spec supports an optional command in the completion results, so
that the server can know what completion candidate was selected.  That
information can then be used by the server to score candidates for
better completion results etc.  This simple patch adds support for this.
I have no strong opinions on _where_ exactly the command should be sent,
as in before or after the didchange notification.  I've been using this
the last couple of days.

What do you think?  Any objections to me installing this sometime later?
Should it be mentioned in NEWS, or isn't it interesting enough?

Theo