Fix position of docstring.
Make all related variables buffer local.
Set indentation by mode.
More indentation settings.
Allow derived mode to use parent mode setting.

Allow derived mode to use the setting of the closest parent mode which
exists in `edconf-indentation-alist`.

E.g., `doctex-mode` doesn't have to add its own setting into the alist.
Instead, it can use the setting of `latex-mode` if they use the same
indent variables.