Include additional template imports to suppress NOTEs

[andrjohns]

Any new packages created by rstantools will receive the following NOTE on CRAN submission:

N  checking dependencies in R code (904ms)
   Namespaces in Imports field not imported from:
     ‘RcppParallel’ ‘rstantools’
     All declared Imports should be used.

Because we add those packages to their package Imports but they only get used in the configure & Makevars, so their namespaces aren't imported from.

This PR resolves that by adding the following imports to the template structure:

importFrom(rstantools, rstan_config)
importFrom(RcppParallel, RcppParallelLibs)

[jgabry]

Thanks! Looks good. Feel free to merge once it passes.

[jgabry]

I think this can close #99