Now in addition to the <layer>/init-<pkg> function there are
- <layer>/pre-init-<pkg> (executed before <layer>/init-<pkg>)
- <layer>/post-init-<pkg> (executed after <layer>/init-<pkg>)
The init function is mandatory, if it is not present then the
package is ignored and not installed.
This mechanism allows soft (implicit) cross layers dependencies
between packages (see company for more info).
It is now possible to remove flycheck from spacemacs layer and
move it to its own syntax-checking layer.
