Allow specifying multiple context sources

[trusch]

It would be usefull to be able to specify multiple context sources. With that we could setup a build which pulls its dockerfile and main application from git but additionally loads for example datascience-models from s3 into the build context.

Another usecase would be to have one git repository with multiple folders (each containing a dockerfile and some context) + a common folder which should be part of every build context.

If this is desired I would be able to implement this by myself and would create a PR.

[AkihiroSuda]

Thanks, PR is welcome
( Although I'm more interested in Knative now #76 )