Added cubicInterpolationMode and changed fill enum

[stockiNail]

Added cubicInterpolationMode and changed fill enum

[benmccann]

@stockiNail this PR will need to be rebased

[stockiNail]

rebased and commited

[benmccann]

Thanks for all your patience on this one

[stockiNail]

Thanks for all your patience on this one

@benmccann no problem at all. Being developer, I love coding (coding is art) but I love a little bit less document..
Having a project which wraps Chart.js, I try to read carefully the documentation to provide all functionalities.
Let me use this thread to add a couple of other items which, in my opinion, we should work on:

• Controller: reading doc, it's not clear which data structure new controller should prepare. The methods documentation is missing, the methods needed to create data (or other) structure. To create a controller you should go to have a look to other implementation. The example in documentation is the simplest one.
• New Axis: see above thought about Controller.
• Default values: I have the feeling there are some other unclear defaults (as I wrote label of dataset on different charts)
• Charts methods: isDatasetVisible() is not documented but it's very important due to the usage of hidden property is deprecated

GREAT LIBRARY! THANKS!