Filter out Fold markers.

It started out when I introduced more and more fold markers to the DebOps project. I used `(((` as marker because it was not highlighted by my editor as being a Jinja2 control sequence. The fold marker as turned out to be not-optimal because it also appears in complex Jinja2 conditions quite often. So @drybjed and me settled on `[[[`. The problem was, however, that we always needed to introduce additional redundancy to the files. Example:

```YAML
---

```

@drybjed and I discussed this and agreed that the best thing would be to patch up `yaml2rst` to filter out the fold marker which would allow it to be directly appended after headings for instance. Example:

```YAML
---

```

With this patch, this input gets converted into:

```reStructuredText
.. vim: foldmarker=[[[,]]]:foldmethod=marker

Default variables
=================

.. contents:: Sections
   :local:

----------------------
  Role configuration
----------------------
```

Can we make this the default behavior or should it be made optional? I limited the regex to only match a very defined set of occurrences.

examples/main.yml

@@ -88,7 +88,7 @@ collectd_plugins_default_config:
     IgnoreSelected true
 
 #
-# Indention of Literal Blocks behind Lists
+# Indention of Literal Blocks behind Lists [[[1
 # -----------------------------------------
 #
 # Code will properly be indented after lists:

yaml2rst.py

@@ -104,6 +104,9 @@ def convert(lines):
         elif line.startswith('# ') or line == '#':
             if state != STATE_TEXT:
                 yield ''
+            # Filter out [[[\d and ]]] at the end of a documentation line.
+            # Those sequences can be used for folding sections.
+            line = re.sub(r"\s*(:?\[{3}|\]{3})\d?$", "", line)
             line = last_text_line = line[2:]
             yield line
             last_indent = get_indent(line) * ' '