Reconsider simplifying our color palette

[dpordomingo]

As described by https://github.com/src-d/code-annotation/pull/53#discussion_r165371511 we could reconsider simplifying our color palette. When I reviewed the PR #53 we had 18 different colors.

Here are the colors I found in our app: (duplicated color, means it is repeated in different places in our stylesheets)

in raw:

#ffffff x4
#d8dcdc x2  
rgba(0, 0, 0, 0.2)
#cccccc x2
#a0a2a2 x2
#979a9a
#818a8a x2
#808b8c x3
#444444
#2c3132 x2
#191b1c
#0c0d0e x2
#00f3ff x2
#66ffba x2
#f37b6f x2
#006166
#004c2a
#61312c