Add ftpackage, reorganize vimrc_after.

Add:
	vimrc
		-Reorganize the `settings` section into boolean/non-boolean
		settings.

	vimrc_after
		-Reorganize contents into an indented hierarchy with sections
		delimited by header comments, like that in `vimrc`.

	vim/ftpackage/
		-Add `ftpackage/`, to contain all `ftpackage` files;
		these are essentially ftplugin files common to several
		`ftplugin/` files.

	vim/ftplugin
		-Move repetitive groups of statements into `ftpackage/` files,
		to be included by individual ftplugin files.