library_call_linter() should exempt header boilerplate in Rmd

[MichaelChirico]

Originally posted by @MichaelChirico in #2152 (comment)

It's very common to start an Rmd vignette with an include=FALSE header setting up metadata:

```{r setup, include=FALSE}
knitr::opts_chunk$set(...)
```

```{r first_visible_cell}
library(my.pkg)
```

But library_call_linter() lints this. The right approach is probably to exclude certain calls preceding the first library().

I've also wondered if commandArgs(TRUE) / options parsing must come after the library() calls.