Add publish date of versions to changelog (#65)

Add the publish date to the changelog for each version so it's easier to
tell when looking at the changelog when a version of a package was
released.

Closes #35

.changesets/add-published-on-date-to-changelog.md

@@ -0,0 +1,6 @@
+---
+bump: "patch"
+type: "change"
+---
+
+Add the publish date for each version to the changelog. This allows people to see at a glance when a version was released.

lib/mono/changeset_collection.rb

@@ -48,10 +48,13 @@ def write_changesets_to_changelog
       contents = File.read(changelog_path)
       lines = contents.lines # Keep original contents to add to the bottom
       heading = lines.shift # Keep original heading
+      date = Time.now.utc.strftime("%Y-%m-%d")
       File.write(changelog_path, <<~CHANGELOG)
         #{heading}
         ## #{package.next_version}
 
+        _Published on #{date}._
+
         #{content.join.strip}
 
         #{lines.join.strip}

spec/lib/mono/changeset_collection_spec.rb

@@ -229,6 +229,8 @@ def map_changesets(changesets)
         expect(changelog).to include(<<~CHANGELOG)
           ## 2.0.0
 
+          _Published on #{date_label}._
+
           ### Added
 
           - [LINK] major - This is a major changeset bump.
@@ -276,6 +278,8 @@ def map_changesets(changesets)
         expect(changelog).to include(<<~CHANGELOG)
           ## 2.0.0
 
+          _Published on #{date_label}._
+
           ### Deprecated
 
           - [LINK] patch - This is a patch changeset bump.
@@ -293,4 +297,8 @@ def normalize_changelog(content)
     # Remove links so we don't have to try and match against every instance
     content.gsub(/\[[a-z0-9]{7}\]\(.+\)/, "[LINK]")
   end
+
+  def date_label
+    Time.now.utc.strftime("%Y-%m-%d")
+  end
 end