Handle complex variable names with backticks

[Ralayax]

Uses backticks to handle complex variable names

Fixes #630

(I'm pretty new to R packages and I don't know a lot about the mice repo so forgive me if this isn't up to standards)

[stefvanbuuren]

When I do usethis::pr_fetch(631) I get

Error in libgit2::git_remote_fetch : unexpected http status code: 404

Not quite sure why that is. Perhaps fetching from a feature branch instead of master might work?

[stefvanbuuren]

Now it works, after solving a typo in the remotes.

Thanks a lot. I ran some examples, and there it worked very well. Although we should not encourage slopping names, it is useful to support them, so I'll merge.