WIP: Group customizations

This is the basic idea that allows the user to customize on a per-group of faces:

    (setq solarized-height-plus-1 1.5) ;; This affects everything
    (setq solarized-org-height-plus-2 1.0) ;; This is just for Org Mode

There is a problem with `solarized-use-variable-pitch` though. It is indirectly used through `s-variable-pitch` which is an ad-hoc variable that contains the real value that will be used on the face definition. We could either:

 * define similar variables for all relevant groups, which is tiresome

 * remove `solarized-use-variable-pitch` and define a new customization (let's say `solarized-pitch`) that would contain the actual value: `variable-pitch`, `fixed-pitch` or `default`. Doing this has the advantages of making `solarized-pitch` consistent with the other customizations (in the sense that the value you set is the value actually used) plus allowing for `fixed-pitch` makes possible for the user to override the default value of the inherited face when it has `variable-pitch`. The obvious disadvantage is that people currently using `solarized-use-variable-pitch` would have to adapt their configuration.