- Move rtags key bindings under `SPC m g` prefix
- Document rtags support and key bindings in README.org
- attempt to implement gtags fallback using
https://github.com/Andersbakken/rtags#fall-back-to-other-taggers
- make c-c++ layer dependend on gtags layer (gtags layer is automatically
installed when the c-c++ layer is used)
- sort packages config in packages.el
- use a dolist instead of a function to define rtags key bindings
