Add support for newlines in MySQL passwords

[tianon]

Closes #233

I verified this by doing the following:

$ docker run -dit --name mysql -e MYSQL_ROOT_PASSWORD=$'\na\nb\nc\n' mysql:5.7
fdc19dd20c8a203c5614272d760c557c49ec3fff8646fe555043862738f46b4f
$ docker exec -it mysql mysql -uroot -p$'\na\nb\nc\n' -e 'SELECT 1'
mysql: [Warning] Using a password on the command line interface can be insecure.
+---+
| 1 |
+---+
| 1 |
+---+
$ docker run -it --rm -e WORDPRESS_DB_PASSWORD=$'\na\nb\nc\n' --name wordpress --link mysql:mysql ee9b41f73de3
...

(Where ee9b41f73de3 is an image built with this change.)

Then I hit my new wordpress container from the browser and went through the setup process.

Without this change, the WORDPRESS_DB_PASSWORD variable having newlines in it causes sed: -e expression #1, char 80: unterminated s' command` (as in #233).

[tianon]

Here's the DB_PASSWORD value this creates:

define('DB_PASSWORD', '' + "\n" + 'a' + "\n" + 'b' + "\n" + 'c' + "\n" + '');