Retry on connection

[athos]

Resolves #2.

This PR adds the --retry-timeout DURATION and --retry-interval DURATION options to control connection retry.
For example, the following command will retry the connection every 5 seconds for up to 30 seconds:

$ trench --retry-timeout 30s --retry-interval 5s

This change breaks the library-level compatibility for the nrepl and prepl clients.
Before the change, to create an nREPL client, you needed to do something like the following:

client := nrepl.NewClient(&Opts{Host: "localhost", Port: 12345, ...})

After the change, you need to do something like this:

client := nrepl.NewClient(&Opts{ConnBuilder: &client.TCPConnBuilder{Host: "localhost", Port: 12345}, ...})

The same goes for the prepl client.