Provide the ability to generate BUILD.bazel file instead of just BUILD

[Shashank Bharadwaj]

I'd like the ability to generate BUILD.bazel files instead of the standard BUILD files.

I feel it would be good to provide an option like:
BUILD_FILE_NAME = 'BUILD.bazel'

in the .pazelrc file to allow this behaviour. This should be an optional argument, by default, of-course we can continue to generate BUILD files.