Add bump and pre params to receive from file

[axpira]

I think it's good to decide to what version to go in a plan.
Than I want to add two new params: bump_file and pre_file
With this I can set this params in another task to pass
And only one of this is need to pass: bump or bump_file and pre or pre_file

Thanks

[vito]

Linking concourse/concourse#684 which would remove the need for this as an explicit feature.