Fix #24: Raise lower bound of `machines-directory`.
Bump version to 0.2.0.4.
Relax constraints on `MissingH` and `text`.
set format emacs: Use --etags and not --ctags for emacs.
Remove redundant imports.
Config: Share migration code (in preparation of adding another config version).

  * Each Codex version module responsible for its own warning and migration
Allow configuration of tags file name.

* emacs format sets tags file name to "TAGS" (this is where emacs
  expects to find tag files)
Release version 0.2.1.0.
Bump to development version 0.2.1.1.