indentation_linter: fenced code blocks inside enumeration in RMarkdown document

[klmr]

Consider the following RMarkdown document, which has fenced code inside an enumeration:

* An enumeration item with code:

  ```{r}
  "Hello"
  ```

‘lintr’ has the spurious complaint

mwe.rmd:4:2: style: [indentation_linter] Indentation should be 0 spaces but is 2 spaces.
  "Hello"
~^

When removing the indent (which works, but is unconventional) and knitting the document, ‘knitr’ reinserts the indent in the output Markdown document.

[AshesITR]

This is somewhat related to #1043.
In general, we should make a more sane method of extracting R source from RMarkdown - in this case it could auto-detect the markdown indentation by seeing the whitespace in front of ```{r} and trim that accordingly.