Make renaming rextendr package easy

[JosiahParry]

If you want to change the name of an rextendr R package the easiest way to do so is to create a whole new package and copy over the source files. However, it would be niceto have a function that will do that for you. Going through all mentions of the original package name and changing it tough.

[daniellga]

That would be very interesting. I also would like to be able to have the choice to update my package to use the last version of rextendr, even If It overwrite some existing files.

[eitsupi]

I am trying to add the ability for use_extendr to overwrite existing files at #292.

Note, however, that this package does not have the ability to read toml, so when updating Cargo.toml, any previously manually updated content will be removed. (Of course, all files such as lib.rs will be initialized)