added lmerTest

[jkylearmstrong]

I have successfully added lmerTest to the package, as I requested in #73.

usethis::use_package('lmerTest', type = "Suggeasts", min_version = TRUE)
devtools::load_all()
devtools::document()
devtools::test() # passed
devtools::install()

usethis::use_version(push = FALSE) # set to 'dev'

devtools::load_all()
devtools::document()
devtools::test() # passed
devtools::install()

[hfrick]

Thanks so much for the PR here! I won't be able to look at it in detail atm but wanted to thank you for the initiative! When I get to it, these dev checklists are what I'll be working through.