Add usage
Add protocol to url in script tag

Make index.html usable on file system
Change keyboard shortcut for changing dubeol/sebeol

In Windows, Alt+Space is assigned to pop up window menu.
Alt+Shift+Space is fine on both Windows and Mac OS.
Changing dubeol/sebeol is expected to be rare, so more keys are
not very harmful.
Correct the way IME key is treated

IME key is common for Korean keyboard, labeled '한/영'. This key
doesn't have to be combined with Shift key.
Bump into 0.2.1
Merge branch 'release-0.2.1'