tests: ensure version numbers in README are always up to date

This adds a test that will check the examples in README.md for the
current package version.

It also checks that there is a Markdown heading mentioning (at least)
the current version. This serves as a simple check that the change log
section is updated when the version number is bumped.

Related to unicode-rs#30, unicode-rs#41, unicode-rs#47, and unicode-rs#48.

Cargo.toml

@@ -23,3 +23,4 @@ no_std = [] # This is a no-op, preserved for backward compatibility only.
 
 [dev-dependencies]
 quickcheck = "0.7"
+version-sync = "0.8"

tests/version-numbers.rs

@@ -0,0 +1,14 @@
+#[test]
+fn test_readme_deps() {
+    version_sync::assert_markdown_deps_updated!("README.md");
+}
+
+#[test]
+fn test_readme_changelog() {
+    version_sync::assert_contains_regex!("README.md", r"^## {version}$");
+}
+
+#[test]
+fn test_html_root_url() {
+    version_sync::assert_html_root_url_updated!("src/lib.rs");
+}