misc/vim: Disable automatic line wrapping by textwidth.

If someone configures a 'textwidth' in go files, vim will by default insert
newlines into long lines as you type, which breaks syntax and doesn't really
make sense for go code. This fixes the default.

LGTM=dsymonds
R=golang-codereviews, gobot, dsymonds
CC=golang-codereviews
https://codereview.appspot.com/76890046

Committer: David Symonds <dsymonds@golang.org>