Merge packages and extensions

[syl20bnr]

This task depends on #110 because we need to retain backward compatibility with the current layers format.
The idea is to have only one list per configuration layer to declare all the packages and extensions.
This list will have convenient properties like:

• :type with possible values melpa, melpa-stable, el-get or quelpa or even submodule (although we definitely don't want them).
• :step with values pre or post to control the loading time.
• :disabled t to mark a package as not activated by default (useful to be able to incorporate contrib layers with only one package in spacemacs layer).

[syl20bnr]

Note to self: check el-get recipes

[tuhdo]

@syl20bnr quelpa is merged into MELPA: melpa/melpa#2968

Maybe it is the time?

[syl20bnr]

Almost one year later it is now implemented in develop!