Add expand_template rule (#330)

Resolves: https://github.com/bazelbuild/bazel-skylib/issues/191

An example of how this can be useful can be found here: https://github.com/catchorg/Catch2/pull/2387/files

Could be also helpful here: https://github.com/AcademySoftwareFoundation/openexr/blob/8587f4eed1d396a9c545d59e7b231d9d4b2cd8ef/BUILD.bazel#L21

rules/BUILD

diff --git a/rules/BUILD b/rules/BUILD
index f7017bb..b89495e 100644
--- a/rules/BUILD
+++ b/rules/BUILD
@@ -33,6 +33,11 @@
 )
 
 bzl_library(
+    name = "expand_template",
+    srcs = ["expand_template.bzl"],
+)
+
+bzl_library(
     name = "native_binary",
     srcs = ["native_binary.bzl"],
     deps = ["//rules/private:copy_file_private"],