Do not always allocate a tty on connect

To import SQL files `connect` has to be used instead of `import`. Doing
so can’t work with the `-t` docker option, so now we check for a tty
before using this option.

Fixes #18

tests/service_connect.bats

@@ -24,6 +24,6 @@ teardown() {
 @test "($PLUGIN_COMMAND_PREFIX:connect) success" {
   export ECHO_DOCKER_COMMAND="true"
   run dokku "$PLUGIN_COMMAND_PREFIX:connect" l
-  assert_output 'docker exec -it dokku.postgres.l psql -h localhost -U postgres l'
+  assert_output 'docker exec -i -t dokku.postgres.l psql -h localhost -U postgres l'
 }