This basic Vim ftplugin sets the 'comments' and 'commentstring'
settings to sensible values. Future filetype settings for Go
would go in the same file.
The ftplugin was added as misc/vim/ftplugin/go/go.vim, this way
the installation instructions in readme.txt remain valid.
Fixes #5715.

R=dsymonds
CC=gobot, golang-dev
https://golang.org/cl/10323043