Implement the export_file command in buildozer

[vladmos]

To add or modify an "exports_files" declaration one may use the following command:

buildozer 'export_file filename.txt //visible/for:__pkg__' //path/to/target

It's also possible to provide no or several visibility arguments, if a corresponding "exports_files" section with the same visibility parameters exists it will be updated.

[laurentlb]

Documentation is missing.
But I question the value of this new command. Is it a common enough need? exports_files should be rarely used in Bazel.