As of today (May 2018), every scripts that you add on Google App Scripts (including scripts on Google Drive like spreadsheets) will get accessible through https://script.google.com and will have a specific id that you can get through the url by opening it.
By using clasp which handle a project (like git), you can do:
clasp clone {id}
to have your script in a local folder.
Then, after editing your file with your favorite text editor, upload it back with :
clasp push