Add context options: -A/-B/-C; Fixes #287 (#722)

* Add context options: -A/-B/-C; Fixes #287

* Fix indentation of line continuation.

* Fix 'only before context' case.

* Add some tests for the new context options.

* Refactor context code (pass context tuple, instead of before/after variables)

* Get rid of 'remove temp file code' in context test.

* Fix reintroduced error (regarding 'only before context').