Emacs go-mode: don't indent the inside of strings.

Emacs go-mode: don't indent the inside of strings.

Two fixes for indentation problems:

1. Properly recognize multi-line strings. These start with `, not ".

2. Don't indent a line if the beginning of the line is the end of a multi-line string. This happened for instance when inserting a closing bracket after a multi-line string.

R=sameer
CC=golang-dev
https://golang.org/cl/6157044