Use proper quoting when handling PROMPT_COMMAND

May fix issue #250.
Add LP_PATH_DEFAULT to allow to customize how $PWD is shown

See feature request #219.
Cleanup previous commit about LP_PATH_DEFAULT

- avoid overriding the existing LP_PATH_DEFAULT from the environment
- use the _LP_*_SYMBOL convention for shell dependent default values
Merge branch 'develop' of github.com:nojhan/liquidprompt into develop

* 'develop' of github.com:nojhan/liquidprompt:
  Cleanup previous commit about LP_PATH_DEFAULT
  Add LP_PATH_DEFAULT to allow to customize how $PWD is shown
  Use proper quoting when handling PROMPT_COMMAND