Add ability to transform multiple selections at once.

The Atom editor has ability  to select multiple lines and then execute action on this multiple selection. This is really useful with encoding functions as you can select only parts of the file that is url encoded and then decode all parts at once and other way around.

I use this feature in sublime quite frequently when working one outdated pice of software where things like this dictionary are popular:

    {
       "key": "very%20long%20url%20encoded%20value",
       "another key": "and%20another%20value%20with%20quote%3A%20(%22)%20character"
    }

Unfortunately the url-encode does not support this case yet, this pull request add this functionality. (fixes #1)