Opens specified text editor when creating a new changelog entry

Signed-off-by: Rémy Coutable <remy@rymai.me>

bin/changelog

@@ -148,7 +148,7 @@ class ChangelogEntry
 
   def execute
     assert_feature_branch!
-    assert_title!
+    assert_title! unless editor
     assert_new_file!
 
     # Read type from $stdin unless is already set
@@ -162,6 +162,10 @@ class ChangelogEntry
       write
       amend_commit if options.amend
     end
+
+    if editor
+      system("#{editor} '#{file_path}'")
+    end
   end
 
   private
@@ -180,6 +184,10 @@ class ChangelogEntry
     File.write(file_path, contents)
   end
 
+  def editor
+    ENV['EDITOR']
+  end
+
   def amend_commit
     fail_with "git add failed" unless system(*%W[git add #{file_path}])