Make work with indented code

[basnijholt]

Either fix or raise, e.g.,

markdown-code-runner/README.md

Lines 68 to 77 in 87837d8

	<!-- CODE:SKIP --> <!-- This is here otherwise the next example gets executed -->
	````markdown
	```python markdown-code-runner
	print('Hello, world!')
	```
	(Optionally, you can place some text between the code block and the output markers)
	<!-- OUTPUT:START -->
	This content will be replaced by the output of the code block above.
	<!-- OUTPUT:END -->
	````

wouldn't work without the SKIP.