Added type property in ChartDataSets to support mixed charts

[portno]

Didn't find any other way to do it.

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[portno]

I signed it!

[googlebot]

CLAs look good, thanks!

[kevmoo]

Which version of chart.js was this introduced in?

[portno]

According to this release it was added in version 2.0.0, Charts that combine visual types such as bars and lines. Also, they mention mixed chart types in their official website as New in 2.0

[kevmoo]

I just pushed an update that should include chart.js 2.6

Please take a look at master and compare...

[portno]

The type property exists. Please close this PR.

I've created another issue with feedback. #22