Add supervisor_options option to specify top-level supervisor options

[mtrudel]

I can't believe this wasn't in the library already, but this adds support for specifying top-level options to the supervisor at top of a Thousand Island server instance. Specifically, this allows you to name a server for easier access (see the docs for more). An example:

iex(1)> ThousandIsland.start_link(handler_module: Echo, supervisor_options: [name: :my_special_name])
{:ok, #PID<0.254.0>}
iex(2)> ThousandIsland.listener_info(:my_special_name)
{:ok, {{0, 0, 0, 0}, 4000}}