lock --reinstall deletes first and clones later #99
Description
It appears the lock --reinstall command will first delete all the existing directories, and then try to re-clone them:
https://github.com/rossmacarthur/sheldon/blob/master/src/lock.rs#L169-L176
This is unsafe, as cloning can fail, which will leave the user without any of their plugins. It should instead clone to a temporary directory, and then once everything is successful, then delete the existing directories and move the temporary ones into place.