Changes to plugin to cater for the following:

1) Supply default values for extension configuration values as required by Markdown 2.5
2) Print exception & error message if extension creation fails
3) Return 'root' from PelicanMathJaxTreeProcessor.run as required by the spec in Markdown 2.5
1) Integrate changes needed for mathjax version 2.5 compatibility
2) Simplified displayed math regex by removing unneeded non-capturing group