Include support for a new <springProperty> element which can be used in
`logback-spring.xml` files to add properties from the Spring
Environment. For example:

	<configuration>
		...
		<springProperty name="destination" source="my.loggger.extradest"/>
		<appender name="FILE" class="ch.qos.logback.core.FileAppender">
			<file>${destination}</file>
				...
			</file>
		</appender>
		...
	</configuration>

Fixes gh-1788