Add pointer to current toppest Parser and current Option to Situation.

Add Circumstance to MatchResult, which can be used to pass option scope information to taker.
Add document

Add Circumstance.set
explosure get of description, epilog and help_level of Parser.
Code clean

ADD_TEST in cmake

Add hide_from_help, set_help_level

Add some tests.

New Circumstance

Import join

Add test of reading/parsing config file.

Add error message of errno when open config file failed.
Refine get_error, add is_error and is_ok
WIP:Delete usages of get_arg and get_match_arg
Simplify codes.

Rename MatchFromDescriptor to MatchDescriptor