Avoid calling syntax-propertize-rules as may be unavailable.

Instead of using the macro syntax-propertize-rules, use the evaluated
macro. Older Emacs may not have the macro available, the final
function is short and easy to understand.

GitHub-Issue: 134
Create a unit test for highlighting interpolated variables

GitHub-Issue: 136
Add more tests for highlighting variable interpolation

Suggested-by: Chris Rasys
GitHub-Issue: 136
Guard propertize functions for Emacs that don't support it.
Merge commit '38e1940e950d47737fed30a5bd5d4e75e0faf103'

* commit '38e1940e950d47737fed30a5bd5d4e75e0faf103':
  Guard propertize functions for Emacs that don't support it.
  Avoid calling syntax-propertize-rules as may be unavailable.
Merge branch 'ejmr/highlight-variables-in-strings'

* ejmr/highlight-variables-in-strings:
  Add more tests for highlighting variable interpolation
  Create a unit test for highlighting interpolated variables
Update version to 1.13
README: Stress that I am looking for a new maintainer