Make preference-opening command universal (#267)

The `$EDITOR ~/.hyperterm.js && exit` requires that the default shell recognize variables as commands and to recognize the `&&` syntax, so it does not work with fish shell. The new command works with all common shells, including fish.

app/lib/actions/ui.js

@@ -131,7 +131,7 @@ export function showPreferences () {
               uid,
               ['# Attempting to open ~/.hyperterm.js with your $EDITOR',
                '# If this doesn\'t work, open it manually with your favorite editor!',
-               '$EDITOR ~/.hyperterm.js && exit',
+               'exec env $EDITOR ~/.hyperterm.js',
                ''
               ].join('\n')
             ));