Improve Mac OS support

[LexiconCode]

This issue tracks Mac support. I don't have access the Mac hardware for testing purposes so I will have to rely on the community testing.

• Shortcuts for copy and paste are different for Mac OS eg. w-c/w-v. The following situations need to be handled anywhere there's a reference to shortcuts c-c/c-v:

  • https://github.com/dictation-toolbox/Caster/blob/master/castervoice/lib/navigation.py
  • https://github.com/dictation-toolbox/Caster/blob/master/castervoice//lib/context.py

• The bring me command won't work as expected until someone with Mac implements exposing clipboard formats in dragonfly.

• Once the above issues are fixed and tested the Mac OS install instructions need to be added.

[LexiconCode]

* Shortcuts for copy and paste are different for Mac OS eg. w-c/w-v. The following situations need to be handled anywhere there's a reference to shortcuts c-c/c-v:

improved in #912 but more work needs to be done