Add simple framework for working with text

The initial version allows for loading text from a file, as well as
providing a new plugin type for text markup formats.
Create a component for common text formats
Add text handler for Markdown format
TextService: add isText(File) convenience method
DisplayPostprocessor: ignore null outputs

In this way, a module can declare multiple potential outputs and only
populate the ones relevant to each particular execution.
Generalize the File Open command

It should not limited to images, but rather work with any input file.
The current implementation should be extracted to a service (perhaps
part of IOService), but works for the moment to add support for text.
DefaultTextService: always wrap in "<html><body>"

This is necessary (at least, the "<html>" prefix is) for Java's HTML
rendering using JEditorPane.
SwingTextDisplayPanel: use a JEditorPane

Unlike with JTextArea, this allows for the rendering of HTML.
ShowReadme: lean on the new TextService
fixup! SwingTextDisplayPanel: use a JEditorPane
Merge branch 'text-formats'

This branch adds support for decoding various text formats into HTML.
Usage of HTML was chosen because Java's JEditorPane supports simple
rendering of HTML (as opposed to syntaxes such as Markdown).

The end result, for the moment, is that ImageJ now displays its
README.md file in (mostly) beautiful HTML, rather than as raw Markdown.
But the hyperlinks don't do anything yet...