Force to keep some packages

[ahasverus]

Feature description

Currently, the function add_deps() automatically removes unused packages from the DESCRIPTION file. But for some packages (e.g. rnaturalearthdata & rnaturalearthhires) this behaviour is not appropriate: these packages are not directly called and they are deleted. This is also true for packages previsously called w/ library() and added to the Depends field. They are deleted if the library() calls are deleted (to reduce redundancy w/ devtools::load_all()).

• Option 1: add an argument clean (if TRUE, purges the DESCRIPION file, default). Otherwise do not remove packages from the DESCRIPTION but just adds new ones.
• Option 2: create an additional hidden file (.rdepsconfig) to list mandatory packages (maybe too complicated to handle). This file can be created automatically for packages called w/ library().