[doclint] Prepare doclint for more checks

This patch refactors doclint so that more checks and more generators
could be added.

This patch:
- Introduces 'Source' class, which holds file content in-memory and
  allows it to be updated.
- Introduces 'Message' class - which is a pair of a text and a type.
  Messages could have either 'error' type or 'warning' type.

utils/doclint/check_public_api/test/01-class-errors/01-class-errors.txt

@@ -0,0 +1,3 @@
+[MarkDown] Non-existing class found: Bar
+[MarkDown] Non-existing class found: Baz
+[MarkDown] Class not found: Other

utils/doclint/check_public_api/test/01-class-errors/doc.md

@@ -0,0 +1,5 @@
+### class: Foo
+
+### class: Bar
+
+### class: Baz

utils/doclint/check_public_api/test/01-class-errors/foo.js

@@ -0,0 +1,2 @@
+class Foo {
+}

utils/doclint/check_public_api/test/01-class-errors/other.js

@@ -0,0 +1,2 @@
+class Other {
+}