Fix a bug in chgems (combined paths added as one in the GEM_PATH)
Prevent the modification of the GEM_PATH and the fish_user_paths when chgems is called more than once in a row to prevent duplicate paths
Cache the default GEM_PATH if it is not already cached and append the new gem directory to that

This will make it possible to have only one "gemset" active at a time
Add the possibility for the user to create a file with a list of "global" gems to install in each new "gemset"