misc/vim: reorganize plugin so it uses ftplugin and syntax

This sets up vim to work out of the box with go programs as long as syntax
highlighting is enabled. Both files must be copied to the vim runtime
directory in order for the file-type detection and syntax loading to work.

R=golang-dev, adg
CC=golang-dev
https://golang.org/cl/1723044

misc/vim/ftdetect/gofiletype.vim

+au BufRead,BufNewFile *.go setfiletype go

misc/vim/readme.txt

+Vim syntax highlighting for Go (http://golang.org)
+==================================================
+
+To install automatic syntax highlighting for GO programs:
+
+  1. Copy ftplugin/gofiletype.vim to the ftplugin directory underneath your vim
+     runtime directory (normally $HOME/.vim/ftplugin)
+  2. Copy syntax/go.vim to the syntax directory underneath your vim runtime
+     directory (normally $HOME/.vim/syntax)
+  3. Add the following line to your .vimrc file (or equivalent):
+    
+     syntax on