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
_lp_source_config: fix quoting for LP_MARK_ variables
Merge remote-tracking branch 'upstream/develop' into develop