add a function to highlight matched text

[neilboyd]

TODO:

• add the function
• add it to the example
• make sure it doesn’t trim opening or closing br
• trim length of text not counting tags - created issue Trim length shouldn't include tags #9
• tests for new function
• documentation of new function
• make it work for Turkish text:
  • add Turkish page to example that demonstrates the error
  • use ToUpperCase instead of ToLowerCase