Generate litr using litr

[jacobbien]

The initial version of litr of course cannot use litr to generate itself. However, every version after the initial version of litr can use a preexisting version of litr to generate it. To set this up, there are two steps:

1. Define an initial stable version of the litr package (the "progenitor") that is written the traditional, non-literate programming way.
2. Rewrite litr using the literate programming approach. This means creating a file create-litr.Rmd from the litr template such that litr::render("create-litr.Rmd") will generate the litr package.

[jacobbien]

Commit 66cc7c9 completes this. The first release, v0.0.1, is step 1 and the second release, v0.0.2, is step 2.