(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.
I think the verb here is "follow" rather than "serve". "serve" sort of implies that it will return you the value of the link, rather than following it. So `follow-links` seems like a better name for this option.