Link URI re: issue 486

The proposed changes address the silent failing of adding a link to an unrecognized URI. They allow the user to correct any potential mistakes made to the initial address through a prompt dialogue. 
I like the idea of creating a reference list of allowed protocols that could be used by the function to check the validity of the URI. Would it be good to keep this list in a separate file so that it could be more easily amended in the future (and/or used by other functions)? If so, where would be an appropriate place to put this? chrome/content/zotero/preferences?