New ftplugin adds Import and Drop commands for Go buffers
in Vim.  These commands ensure that the provided package is
imported (or not imported) in the current Go buffer, using
proper style and ordering, without moving the cursor.

E.g.

    :Import strings
    :ImportAs . strings
    :Drop strings

Two mappings are also introduced to help with the fmt package:

    \f => :Import fmt
    \F => :Drop fmt

R=adg
CC=golang-dev
https://golang.org/cl/4009043