PluginFileMerger support for multi-line property values

I ran into an issue today when adding new packages to the *felix.org.osgi.framework.system.packages.extra* property from a static config plugin. I discovered that the PluginFileMerger class was not handling multi-line property values properly. Here is how an overridden block looks before my changes. Note, this is intentional condensed.

    ## FELIX CONFIGURATION
    ## OVERRIDE:com.mypackage.config
    #felix.org.osgi.framework.system.packages.extra=\
    com.dotmarketing.business.web,\
    com.dotmarketing.business,\
    com.dotcms.spring.web,\
    javax.servlet.resources,\
    ## OVERRIDE:com.mypackage.config
    #javax.servlet;javax.servlet.http;version=2.5

This has the result of causing the OSGi Framework not to start. Most likely due to the dangling '\' character? This pull request adds support for multi-line property values to the PluginFileMerger. After these changes, the above block is instead output as follows.

    ## FELIX CONFIGURATION
    ## OVERRIDE:com.mypackage.config
    #felix.org.osgi.framework.system.packages.extra=\
    #com.dotmarketing.business.web,\
    #com.dotmarketing.business,\
    #com.dotcms.spring.web,\
    #javax.servlet.resources,\
    #javax.servlet;javax.servlet.http;version=2.5

I tried to retain a similar code style and to only change the bare minimum. I didn't see any unit tests for this class so my testing is pretty limited. I only really verified my specific use case with the *felix.org.osgi.framework.system.packages.extra*. So, I may have missed something obvious to you all.