pandoc-include crash when the first line of some code is indented

Consider the following MWE:

```
    I'm indented!
I'm not.
```

pandoc compiles it just fine to

<pre><code>    I&#39;m indented!
I&#39;m not.</code></pre>

but with the latest update, pandoc test.md --filter pandoc_include.py -o test.html crashes with

Traceback (most recent call last):
  File "pandoc_include.py", line 272, in <module>
    main()
  File "pandoc_include.py", line 268, in main
    return pf.run_filter(action, doc=doc)
  File "/home/caubert/.local/lib/python3.7/site-packages/panflute/io.py", line 224, in run_filter
    return run_filters([action], *args, **kwargs)
  File "/home/caubert/.local/lib/python3.7/site-packages/panflute/io.py", line 205, in run_filters
    doc = doc.walk(action, doc)
  File "/home/caubert/.local/lib/python3.7/site-packages/panflute/base.py", line 264, in walk
    ans = list(chain.from_iterable(ans))
  File "/home/caubert/.local/lib/python3.7/site-packages/panflute/base.py", line 262, in <genexpr>
    ans = ((item,) if type(item) != list else item for item in ans)
  File "/home/caubert/.local/lib/python3.7/site-packages/panflute/base.py", line 259, in <genexpr>
    ans = (item.walk(action, doc) for item in obj)
  File "/home/caubert/.local/lib/python3.7/site-packages/panflute/base.py", line 275, in walk
    altered = action(self, doc)
  File "pandoc_include.py", line 251, in action
    includeType, name, config = is_code_include(elem)
  File "pandoc_include.py", line 80, in is_code_include
    value, name, config = is_include_line(new_elem)
  File "pandoc_include.py", line 48, in is_include_line
    firstElem = elem.content[0]
  File "/home/caubert/.local/lib/python3.7/site-packages/panflute/base.py", line 106, in content
    return self._content
AttributeError: 'CodeBlock' object has no attribute '_content'
Error running filter pandoc_include.py:
Filter returned error status 1

This was not the case before this update, and can be resolved by removing the indentation from the first line.

DCsunset / pandoc-include

pandoc-include crash when the first line of some code is indented #19