reticulate output in R chunks not captured in R Markdown

[kevinushey]

To reproduce, try rendering the following document (repro'ed with macOS 10.13.6 + R 3.5.1):

---
title: "Reticulate"
output: md_document
---
  
```{r}
reticulate::py_run_string("print('hello')")
```

You'll see that the printed output (hello) is not captured and reflected in the generated Markdown document.

I think the issue here is that knitr attempts to capture R output with a sink(); however, because we don't always install redirects for Python output back to R, that output is lost. See e.g.:

reticulate/R/output.R

Lines 3 to 11 in 82b2eb8

	remap_output_streams <- function() {
	output <- import("rpytools.output")
	output$remap_output_streams(
	write_stdout,
	write_stderr,
	tty = interactive() || isatty(stdout()),
	force = is_windows() && !is_python3()
	)
	}

The fix here would likely be to ensure that the output streams are remapped when we're running reticulate within knitr -- or perhaps we should just always unconditionally remap the output streams.

See also: rstudio/rstudio#3271