This is enabled in the dev build: http://www.sublimetext.com/dev
However, if you don't want to step up to the dev build, you can use this keymapping:
{ "keys": "#"], "command": "insert_snippet", "args": {"contents": "#{${1:$SELECTION}}$0"}, "context":
{ "key": "selection_empty", "operator": "equal", "operand": true, "match_all": true },
{
"operand": "string.quoted.double.ruby",
"operator": "equal",
"match_all": true,
"key": "selector"
}
]
}
(Place in your Preferences > Keybindings - User)
EDIT: added @castles change.