Support for dplyr::join_by()

[etiennebacher]

@beniaminogreen I already added support for join_by() in another package so I figured it would be easier for me to do it here. I hope this is fine.

This PR:

• adds support for join_by() both for Euclidean and Jaccard joins (with some tests)
• reduces duplication of arguments description by using @inheritParams. I think all arguments in each family are the same, the only thing that changes is the function name, I hope I didn't miss anything
• adds a small modif to document the "PACKAGE" object due to update of roxygen2
• adds myself as contributor

Close #93

[beniaminogreen]

This is fantastic! Thanks for your contribution, and for writing tests around it! Merging in now.