anchors: Add new rule to detect undeclared or duplicated anchors

According to the YAML specification [^1]:

- > It is an error for an alias node to use an anchor that does not
  > previously occur in the document.

  The `forbid-undeclared-aliases` option checks that aliases do have a
  matching anchor declared previously in the document. Since this is
  required by the YAML spec, this option is enabled by default.

- > The alias refers to the most recent preceding node having the same
  > anchor.

  This means that having a same anchor repeated in a document is
  allowed. However users could want to avoid this, so the new option
  `forbid-duplicated-anchors` allows that. It's disabled by default.

- > It is not an error to specify an anchor that is not used by any
  > alias node.

  This means that it's OK to declare anchors but don't have any alias
  referencing them. However users could want to avoid this, so a new
  option (e.g. `forbid-unused-anchors`) could be implemented in the
  future. See https://github.com/adrienverge/yamllint/pull/537.

Fixes #395
Closes #420

[^1]: https://yaml.org/spec/1.2.2/#71-alias-nodes

docs/rules.rst

@@ -14,6 +14,11 @@ This page describes the rules and their options.
    :local:
    :depth: 1
 
+anchors
+-------
+
+.. automodule:: yamllint.rules.anchors
+
 braces
 ------