Adds addin to add cran link

[RMHogervorst]

If applied, this commit will create an extra remedy function: package linker. This allows you to select a piece of markdown and turn it into a link to the CRAN page.

example:

• the package remedy is used for...
• select dplyr and use the packager() function
• the package [{remedy}](https://CRAN.R-project.org/package=remedy) is used for..

I also added a test, but I'm not sure I'm using the helperfunctions correctly.

I didn't add a shortcut, because frankly I wouldn't know which one to use.