v0.5.0

• Added text2qti_tk executable, which provides a basic graphical user
  interface (GUI) via tkinter. Added build scripts in make_gui_exe/ for
  creating a standalone GUI executable under Windows with PyInstaller (#27).
• In executable code blocks, .python now invokes python3 on systems where
  python is equivalent to python2 as well as on systems that lack a
  python executable. The README now suggests using .python3 and
  .python2 to be more explicit (#22).
• Added a new keyword argument executable for executable code blocks, which
  allows a custom executable (including path) to be specified for running
  code. Added support for periods in code block language attributes, so that
  things like .python3.8 are now possible.
• Fixed bug caused by swapped identifiers in QTI XML (#18, #19). Now quiz
  descriptions work with Canvas and question titles appear in Canvas in the
  quiz editor (but not in the student view). The following options now work
  with Canvas: Shuffle answers, Show correct answers,
  One question at a time, and Can't go back.
• Installation (setup.py) now requires markdown >= 3.2 to ensure
  compatibility and avoid Markdown parsing bugs fixed in 3.2 (#31).
• Fixed bug that produced incorrect QTI output paths when using quiz files
  outside the current working directory (#28, #29).
• README now covers more options for installing the development version (#20).