1. Private layers

It is now possible to reload the dotfile and layers anywhere with `SPC f e
R`. This key binding **replaces** the old `C-c C-c` and `SPC m c c` only
available in the dotfile.

**Important** You should replace your `(defvar <layer>-packages ...)`,
`(defvar <layer>-pre-extensions ...)`, `(defvar <layer>-post-extensions ...)`
and `(defvar <layer>-excluded-packages ...)` by `(setq ...)`. This will
allow to install newly added packages to a layer without restarting spacemacs
by pressing `SPC f e R`. Be sure to remove the docstring when replacing the
`defvar` since `setq` does not take a third argument. 

2. Yasnippet

Yasnippet has been moved to the layer `auto-completion`.