Implement ScriptIOPlugin

[hinerm]

The ScriptIOPlugin exists as a shell right now. Its open method needs to be implemented:
https://github.com/scijava/scijava-common/blob/master/src/main/java/org/scijava/script/io/ScriptIOPlugin.java#L70

Currently this causes problems because the supportsOpen method is implemented, and is thus misleading.

[hinerm]

comments from ctrueden:
(11:17:36 AM) ctrueden: My initial idea was that the Script Editor should probably be a DisplayViewer. Though every time we have to create new Displays it rubs the wrong way since it is too complex right now.
(11:17:54 AM) ctrueden: The thing is, we need something like ScriptService#show(String script)
(11:17:59 AM) ctrueden: But... what would it do? Publish an event, I think.
(11:18:09 AM) ctrueden: And then something in imagej-ui-swing needs to pick up that event.
(11:18:30 AM) ctrueden: Requires a little careful thought. Probably should file a GitHub issue for it.
(11:19:01 AM) ctrueden: I think we should file that issue and then link it from the bugzilla and then not worry about it anymore today.

[hinerm]

As a quick fix we could make supportsOpen always return false.