Specify file type for buildifier explicitly

[avdv]

When formatting Bazel files, buildifier needs to know the concrete file
type which it usually deduces from the file name which is impossible
when operating on stdin.

For example, the order of load statements in a WORKSPACE file is
significant and re-ordering might break it. (see bazelbuild/buildtools#1268)

[raxod502]

If there's an option in this tool to supply the filename as a command-line argument, we can do that instead to avoid having to re-implement the inference ourselves. Merging this for now as an incremental improvement.

[avdv]

If there's an option in this tool to supply the filename as a command-line argument, we can do that instead to avoid having to re-implement the inference ourselves. Merging this for now as an incremental improvement.

Currently, there is no such option for buildifier. Thanks for merging!