feat: allow to define multiple todo keyword sequences

[seflue]

They can be defined in the config or within an org file.

Summary

This PR adds the ability to define multiple todo keyword sets as described in Orgmode manual.

Related Issues

Relates to #250, #157, PR #956

Closes #250

Changes

• additionally to a single keyword sequence org_todo_keywords allows to define a table of keyword sets
• an org file can have multiple #+TODO: directives
• if multiple keyword sets are defined, either in the config or in the current org file, org_todo keymap triggers fast access mode to select a keyword
• org_todo_prev behaves like S-RIGHT in Emacs Orgmode

Falling back to fast access mode when multiple sets are defined is a bit of a shortcut to get a first version of this feature out of the door. Emacs Orgmode defines some additional keybindings to switch between keyword sets. This is a bit more elaborated and could be implemented in a further PR.

Checklist

I confirm that I have:

• Followed the
  Conventional Commits
  specification (e.g., feat: add new feature, fix: correct bug,
  docs: update documentation).
• My PR title also follows the conventional commits specification.
• Updated relevant documentation, if necessary.
• Thoroughly tested my changes.
• Added tests (if applicable) and verified existing tests pass with
  make test.
• Checked for breaking changes and documented them, if any.

[seflue]

@kristijanhusak It seems, that the indentation test is a bit flaky (those are the both failing test). It also failed occasionally locally on my machine, but that is unrelated to my changes.