Add `ConfigFile` for evaluating Sanford config files

This adds the logic for evaluating Sanford config files. This will
be used to "choose" which server you want to run via the Sanford
CLI. These files work in a similar manner to Rack's rackup files.
The file expects `run` to be called with a Sanford server. The
CLI will then use the server to either start a process or send
a signal to an existing process.

This also renames the `Server` in `test/support` from `TestServer`
to `AppServer`. This is to avoid collisions with the usage of
`TestServer` in unit tests that rely on a server. I don't want to
accidentally couple these unit tests to this server or alter the
server so that the system server tests start breaking in strange
ways.

@kellyredding - Ready for review.