title: logback using external properties file not working correctly
body: <p>I think I hit a playframework - logback specific bug, perhaps not, any assistance would be great! </p>
  
  <p>I' ve needed an email appender to send me emails when an error occures.<br>
  So I created a simple one with all the properties defined within:</p>
  
  <pre><code>&lt;appender name="EMAIL" class="ch.qos.logback.classic.net.SMTPAppender"&gt;
      &lt;smtpHost&gt;myhost&lt;/smtpHost&gt;
      &lt;smtpPort&gt;25&lt;/smtpPort&gt;
      &lt;STARTTLS&gt;true&lt;/STARTTLS&gt;
      &lt;username&gt;username&lt;/username&gt;
      &lt;password&gt;pass&lt;/password&gt;
      &lt;to&gt;to&lt;/to&gt;
      &lt;from&gt;from&lt;/from&gt;
      &lt;subject&gt;Error: %logger{20} - %m&lt;/subject&gt;
      &lt;layout class="ch.qos.logback.classic.PatternLayout"&gt;
          &lt;pattern&gt;%date %-5level %logger{35} - %message%n&lt;/pattern&gt;
      &lt;/layout&gt;
  &lt;/appender&gt;
  </code></pre>
  
  <p>This worked fine.<br>
  Then I wanted to refactor all of the properties cause obviously i don't want to commit passwords and stuff like that to the source control.<br>
  So i replaced all the properties by ${propname} and added this at the begining</p>
  
  <pre><code>&lt;property file="conf/mail.properties"/&gt;
  </code></pre>
  
  <p>now emails are not sent anymore.<br>
  I debugged and printed out the properties and saw that the problem has to do with quotes.<br>
  my to/from are email addresses, when I put them in my properties file I need to surround them with quotes otherwise i get an error like this:  </p>
  
  <pre><code>Caused by: com.typesafe.config.ConfigException$Parse: /myconfdir../mail.properties: 19: Reserved character '@' is not allowed outside quotes
  </code></pre>
  
  <p>Also my password has a "+" symbol in it and if it's unquoted in the properties file i get the same error.<br>
  When debugging with these quoted values I see that the quote is included inside them.<br>
  So an email is trying to be sent to "a@b.com" and no a@b.com.
  Same for from field and for the password.<br>
  So obviously it doesn't work.  </p>
  
  <p>Any idea how to prevent this?</p>