Allow launching standalone backended services

[theindexer]

I'm trying to launch a service in dev mode or in prod mode. They are sufficiently different modes as to have two different build steps.

I could have two named targets, but personally I'd like to do -b service:dev or service:prod.

If there's another existing or proposed solution for this, I'd be down for that too.

Desired behavior:

$ edward start -b myservice:local myotherservice:local
$ <OK_MESSAGE>

Current behavior:

$edward start -b myservice:local myotherservice:local
$ Error: At least one service or group must be specified

Workaround:

$edward start -b myservice:local myservice
$ <OK_MESSAGE>
$edward start -b myotherservice:local myotherservice
$ <OK_MESSAGE>