Adds a new parameter "--list-releases"

Add a new parameter "--list-releases" for the script regression.py, permitting to display all available firefox releases.
As Julien mentioned in the bug, it would be nice to align the colons here. This is optional, but I don't think it should be too difficult (figure out the maximum length of the release string, then pad with spaces while iterating if the current string is shorter than the maximum).