(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.
@cprice404 Both these tests and the ones above are testing the `:follow-symlinks` option. However, the tests above are testing the functionality when the option is set when explicitly adding a context handler yourself, and these tests are testing that that same functionality is shown when specifying the option in the trapperkeeper config.

I can remove one of the sets of tests if keeping both is too much coverage.