Change "header" to "heading" across the library

This should be backward compatible, as all "header" aliases are still
available, though documented as discouraged for future use.

test/headings_with_spaces_at_the_beginning.md

@@ -0,0 +1,20 @@
+Some text
+
+ # Heading 1 {MD023}
+
+ Setext style fully indented {MD023}
+ ===================================
+
+ Setext style title only indented {MD023}
+=========================================
+
+* Test situations in which MD023 shouldn't be triggered.
+
+  ```rb
+  # This shouldn't trigger MD023 as it is a code comment.
+  foo = "And here is some code"
+  ```
+
+* This is another case where MD023 shouldn't be triggered
+  # Test {MD023} Valid heading for CommonMark (see section 5.2)
+    # Test {MD023} Also valid heading for CommonMark