Add transmission network to visualisation vignette

[joshwlambert]

This PR adds information to the data visualisation (vis-linelist.Rmd) vignette on how to subset the contacts table to only includes infected individuals and plot the resulting transmission network using {epicontacts}. This addition includes tabs for subsetting with base R and tidyverse, and so takes on {dplyr} as a suggested dependency.

[joshwlambert]

I've set library_call_linter = NULL in .lintr as I do not want all library() calls to be at the top of the vignette when functions called from another namespace is only used in a single tab. As is the case for vis-linelist.Rmd.