Add quiet attribute

`nixpkgs_package` can now optionally be quiet, like it is in v0.6.
This is useful for users of Bazel older than v3.0 who pipe the output
of `bazel query` to scripts and don't want workspace rules to pollute
the output of the query.

Closes tweag#85

nixpkgs/nixpkgs.bzl

@@ -162,7 +162,7 @@ def _nixpkgs_package_impl(repository_ctx):
             failure_message = "Cannot build Nix attribute '{}'.".format(
                 repository_ctx.attr.attribute_path,
             ),
-            quiet = False,
+            quiet = repository_ctx.attr.quiet,
             timeout = timeout,
         )
         output_path = exec_result.stdout.splitlines()[-1]
@@ -202,6 +202,7 @@ _nixpkgs_package = repository_rule(
         "build_file": attr.label(),
         "build_file_content": attr.string(),
         "nixopts": attr.string_list(),
+        "quiet": attr.bool(),
         "fail_not_supported": attr.bool(default = True, doc = """
             If set to True (default) this rule will fail on platforms which do not support Nix (e.g. Windows). If set to False calling this rule will succeed but no output will be generated.
                                         """),