Show icons when displaying source buttons

When showing files as sources, dragon will now include the default icon
for the file type on the button as well as the name.
Set stdout to line buffered for scripting -t

When stdout was not a terminal the default buffering made it hard to
script the -t option usefully. With this commit, output is always line
buffered, so each URL dragged on will be printed immediately.
Add simple --target example

The example downloads any URLs dragged onto the target using wget.