Document output for roads

This PR documents the output fields from roads() and provides links to relevent census documentation.

This PR:

adds markdown support to Roxygen
documents columns from roads and inherits those for primary_roads() and primary_secondary_road()

Important notes:

using markdown will modify all other .Rd files in the man folder by converting things like my_var to \code{my_var} for proper rendering.

walkerke / tigris