Use Pharo-like colors for {[()]}'s, close #10

I'm not sure if this is perfect, but I like it more than what was there
before, I like how it's synced with another existing Rainbow Parens, and
I really didn't like they undistinguishable grey cycle (236,238,240…).
(cherry picked from commit 408db9276ac9f76b5dae3e79e2b34311e443cd01)
Explain some of the features you get for "free"

(cherry picked from commit ec5e2f7c2f2341bfd156689ebb15fa84e9238de7)

Conflicts:
	README.md
	README.md.erb