Add LogEvent.getContextMap(String) to support easy ports from Log4j version 1.2's getProperty(String) method. Since LogEvent.getContextMap() can return null, it is cumbersome to use since a null check is required for bullet-proof code, adding this 1-arg String method addresses the issue cleanly. getContextMap(String) is not a great method name, but getContextMapValue(String) seems weird and Log4j 1.2's getProperty(String) name does not fit.

git-svn-id: https://svn.apache.org/repos/asf/logging/log4j/log4j2/trunk@1595279 13f79535-47bb-0310-9956-ffa450edef68