Support custom headline keywords

[amake]

          > @shelverizr Can you check https://github.com/amake/org_parser/pull/15 to confirm that that's what you had in mind?

@amake Not entirely, I want to switch from any keyword to any other keyword.

For example, I have two categories of tasks, to-do tasks and completed tasks, their keywords are:

To-do tasks: TODO, NEXT, WAITING, MAYBE Completed tasks: DONE, FAILED, SUCCESS, STUCK, SHUNT

Sometimes I need to switch from NEXT to TODO Sometimes I need to switch from WAITING to MAYBE Sometimes I need to switch from STUCK to NEXT

Originally posted by @shelverizr in https://github.com/amake/org_parser/issues/14#issuecomment-1975495377