Support migration options in web binary (#266)
Support migration options in web binary (#266)

* Support migration options in web binary

This moves the logic to handle migration commands from
(*atccmd.ATCCommand).Execute() to (*atccmd.ATCCommand).Runner(),
so that in case of the migration options being present, a new runner
is returned, which will only handle the migration.

Closes concourse/concourse#2163.

concourse/concourse,concourse/concourse,concourse/concourse,concourse/concourse,concourse/concourse