[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.

.eslintignore

@@ -1,3 +1,3 @@
 third_party/*
 examples/*
-utils/doclint/test/
+utils/doclint/check_public_api/test/