recognize javascript: strings

Just a quick fix up before the next release. Another hack like the `mailto:` thing, but it gets the job done.

Long term, we need to be less specific about various protocols. We should find a way to get this information out of launch services via Objective-C:

    /System/Library/Frameworks/CoreServices.framework/Versions/A/Frameworks/LaunchServices.framework/Versions/A/Support/lsregister -dump | grep 'bindings:.*:'

Then, instead of checking specific prefixes, we can just split on `:` and see if the first component is on the list.