Add argument to specify minimal package versions

[markusdumke]

There should be an argument that allows to add minimal required package versions. E.g. the package versions currently installed on the package authors computer.

[scottkosty]

Thanks for working on this nice package!

It would be nice if somehow we could specify the minimal version number in the @importFrom roxy tag. However, I'm not sure this can be done. If not, then why not create a new 'tag' that is not parsed by roxygen2. For example:

# @importMinVer packagename 1.0.0

Note that it is prefixed with # and not with #' so it is not parsed by roxygen2. This tag could be parsed by pkghelper.