Adds the spacemacs|init-layer-company helper to set up company
completion for a language layer and replaces existing simple company
initializations. More complex initializations should use the new
architecture directly and remain untouched by this commit.
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.
