WIP - custom knit: handle xaringan::inf_mr

[ElianHugh]

What problem did you solve?

Solves #796.

Both knitting and previewing xaringan slides should work as expected

(If you do not have screenshot) How can I check this pull request?

Knitting the following should work (i.e., progress bar signals completion), and closing the webview should dispose of the process.

---
title: "Test"
knit: xaringan::inf_mr
output:
  xaringan::moon_reader:
    lib_dir: libs
    nature:
      highlightStyle: github
      highlightLines: true
      countIncrementalSlides: false
---

class: center, middle

# Test

### 1234

[ElianHugh]

i'm a little iffy on how broad the url regex should be, because i don't want it to trigger on just any url that might pop up in the output stream