(TK-73) Add option to serve symlinks

Add an option to serve symbolic links when serving static
content. This option defaults to false and can be specified
with `add-context-handler` or when specifying static content
in a trapperkeeper config file.
OK, just to validate my understanding of how this is working: you didn't have to change the service protocol function signature because you were able to simply add this option to the options map.  We are changing the signatures of this function in `core`, but we're able to do that without breaking backwards compat with the external API.  That sound correct?