0.1.7

• Update to cmark-gfm 0.28.3.gfm.20.

Also pulls in upstream changes:

cmark 0.5.6.1 (11 Mar 2019):

• Fix CString memory leaks (Anders Kaseorg). The renderer functions return
  a malloc’d string which the caller is expected to free.

• Fix use-after-free bugs (Anders Kaseorg). useAsCString frees the
  CString after the IO action finishes executing, so passing return as that
  action can never be correct. To make sure the CString gets freed at the
  right time, we need a “with”-style abstraction, not just a conversion
  function. While we’re here replace this with withCStringLen, which also
  makes fewer copies.

• Show node type on error for unknown node type.