R package stringer to stringr

MacOS users cannot write to system Gems directory

This can be solved by installing with `sudo`, but the better approach is to
do user install.
Solution based on [Stack OVerflow solution](https://stackoverflow.com/a/38259128)

Close swcarpentry/lesson-example#165

that is missing formating.

Close swcarpentry/lesson-example#195

Add spacebar to style

Document minimal Python version

For https://github.com/swcarpentry/styles/issues/232

Update 05-rmarkdown-example.Rmd

adds key points that are related to frequently used commands

Adding How to add R packages

Rebase of (#186) Assorted minor fixes to spelling, whitespace, hyperlinks & sorting

Some commits from (lesson-example/186) Assorted minor fixes to spelling, whitespace, hyperlinks & sorting

https://cran.r-project.org/doc/manuals/r-patched/R-exts.html#Specifying-URLs

fix github meta data

Fix typos

Updating to handbook as canonical source for CoC

don't break words in code

Update repo_check.py with new GitHub labels

Delete 08-maintenance.md