feature: configurable vertical segments

[sudodoki]

Add ability to create vertical grid.
Screenshot of resulting chart:

With next element added to config object:

...
        verticalGrid: {
          condition: function(index) {
            return index % 2
          },
          color: '#888',
          opacity: '0.2'
        }
...

[oesmith]

Very cool.

Same feedback as before: could you keep the options namespace flat? Thanks!

[sudodoki]

Will work on this as soon as I get some time on my hands. Thanks for feedback. Should I also add example page?

[sudodoki]

Update PR. Will be doing an example for this one as separate PR.

[sudodoki]

In fact. @oesmith, I would love to become a contributor, even if my only help will be with sorting/labeling issues, reviewing PRs, etc. I do understand your position on keeping Morris clean and simple so I wouldn't be pushy around new feature (like #149) though I'd really like to see that and I do value efforts you and others spent to make Morris what it is.