Improve Monokai highlight style to match original

The current monokai style in highlightjs is not very true to the
original and lacks colors for certain syntactic items. This change's
goal is to bring the highlightjs monokai style in line with the original
design from http://www.monokai.nl/blog/2006/07/15/textmate-color-theme/
Merge branch 'improved-monokai' into 'master'

Improve Monokai highlight style to match original

The current monokai style is not very true to the original and lacks colors for certain syntactic items as mentioned in ticket https://github.com/gitlabhq/gitlabhq/issues/6855. This change's goal is to bring the monokai highlight style in line with the original design from http://www.monokai.nl/blog/2006/07/15/textmate-color-theme/

Before:
![monokai-old](https://gitlab.com/uploads/klowner/gitlab-ce/ce96c7d7c2/monokai-old.png)

After:
![monokai](https://gitlab.com/uploads/klowner/gitlab-ce/b257a8143a/monokai.png)

See merge request !238