Add keyboard shortcut to apply highlighting
Wax should have a keyboard shortcut for applying highlighting to selected text.
Juliana had suggested a toggle on/off feature where the cursor becomes a highlighter that highlights on click-and-drag until the highlighter is toggled off again. This would be used to make many highlights easily. A keyboard shortcut is simpler to implement but would also allow fast highlighting.
This relates to #172 in that once the highlighter can retain the last applied color, this shortcut should use the last applied highlight color as well.