Fix discrepancy between doc and code on per-project settings

 * make working_directory effectively optional
 * make lint_args correctly override default args
 * clarify doc
Bumping beta version number.
Merge branch 'notsqrt-lintargs' of https://github.com/NotSqrt/SublimeLinter into NotSqrt-notsqrt-lintargs

* 'notsqrt-lintargs' of https://github.com/NotSqrt/SublimeLinter:
  Fix discrepancy between doc and code on per-project settings
Merge branch 'master' into dev

* master:
  Bumping beta version number.
Merge branch 'NotSqrt-notsqrt-lintargs' into dev

* NotSqrt-notsqrt-lintargs:
  Fix discrepancy between doc and code on per-project settings