Work out how to set custom lintr parameters in package structure

[cjrace]

Currently we have a note flagging that the .lintr file shouldn't exist, we're using this as the config to set custom lintr parameters.

CRAN guidance suggests this kind of file shouldn't be in the root of the package so we'll need to find an alternative way (or bite the bullet and be strict on the 80 character limit for all lines) - https://cran.r-project.org/doc/manuals/R-exts.html#Package-structure-1