Replace removed global rainbow delimiters mode with custom hooks

See: https://github.com/jlr/rainbow-delimiters/issues/40
Remove rainbow-delimiters mode

Useless without font-lock.
Log when automatically installing a package
Remove unnecessary command specialization
Rename packages initializer
Move elisp to subfolder to avoid adding .emacs to load path
Rename customization file to match convention

Adding the "-custom" suffix makes conflicts on load path unlikelier.