Banner not correctly prepended by concat:js

Not sure if "<banner:meta.banner>" syntax from within a file-src
directive is an old grunt-contrib-concat feature, or if it's a typo and
never worked. Either way, the banner for the concat:js task is now read
from concat:js options.

I decided to make the banner option a target-level option rather than
task-level since the same banner would not likely be used for any
additional targets a user might add themselves. This way they wouldn't
have to override the banner for each of their custom targets.