Highcharter sparklines in DataTables: proof of concept

First of all, I know this issue is related to #132 and #141, but this is specific to **inserting a Highchart object in a rendered DataTable. [Watch the demo in action.](https://nuno-agostinho.shinyapps.io/Highcharter_sparklines_in_DataTables/)**

Highchart objects can't be rendered inside DataTables so this is what I did:
- Create an Highchart object as usual (you can do almost whatever you want with it, except using JavaScript expressions) with the addition of the function `hc_new_sparkline()` to group data series
- Convert the Highchart object to many HTML elements containing the Highchart's JSON code
- Include one HTML element for each row of the DataTable
- Every time the DataTable is drawn (for instance, when it changes pages), it calls a callback function that convert the JSON code in the HTML elements to interactive Highcharts

This allows to use all the advantages of DataTables with Highcharts. So, what do you think @jbkunst?

I put the code available in here if you want to take a look: https://github.com/nuno-agostinho/Highcharter-in-DataTables

Some of this was based on http://www.highcharts.com/demo/sparkline.

Start by reviewing the linked Highcharter-in-DataTables proof of concept and the related issues #132 and #141. The issue does not identify repository files, tests, or a defined completion criterion; maintainers would need to decide whether and how this proof of concept should become project functionality.

Written by the indexing model from the issue text.