Add completion for jobs and variable files.

[jmcarp]

cc @chendrix

[andrewedstrom]

@jmcarp This looks great and worked when we tried it locally, after we set up completion like this: http://godoc.org/github.com/jessevdk/go-flags#hdr-Completion

Before we merge these changes, would you please write a test for the variable files completion?

[jmcarp]

Thanks for reviewing @andrewedstrom ! It turns out that go-flags fixed the issue that required a custom slice type for slice completions the other day, so I reverted the commit for variable files completion and filed concourse/concourse#1383.

[jtarchie]

@jmcarp, thanks for the PR!

@andrewedstrom, sorry for I accepted the PRs in the wrong order. Didn't mean to take the satisfaction of pressing that button.