Install, uninstall, and pathlist

mpm now looks for files called install.m when installing, uninstall.m when uninstalling. It runs these commands after confirming with the user (or you can add the --force flag to auto-confirm).

mpm also looks for a file called pathlist.m when installing, and adds the contents of this file to the path. This provides a way for packages to be explicit about which subfolders should be added to the user's path when installing.

Finally, File Exchange urls now have a new format, so this version will handle that.