Make bar styling options scriptable

[simonbrunel]

The bar backgroundColor, borderColor, borderWidth and borderSkipped options are now scriptable (unit tests, docs and a basic sample). Also fix the gulp task that generates the documentation on Windows.

For example:

backgroundColor: function(ctx) {
    var v = ctx.dataset.data[ctx.dataIndex];
    var c = v < -50 ? '#D60000'
        : v < 0 ? '#F46300'
        : v < 50 ? '#0358B6'
        : '#44DE28';

    return Color(c).alpha(1 - Math.abs(v / 150)).rgbString();
}

borderColor: function(ctx) {
    var v = ctx.dataset.data[ctx.dataIndex];
    return v < -50 ? '#D60000'
        : v < 0 ? '#F46300'
        : v < 50 ? '#0358B6'
        : '#44DE28';
}

Fixes #5302

[simonbrunel]

About the CodeClimate issue, I'm going to see how to refactor it when introducing scriptable options for the line chart, which is a bit different because of the inconsistent option naming between the dataset and the options.elements.

[benmccann]

Lgtm. I think there's one unused variable, but otherwise feel free to merge

[simonbrunel]

@benmccann @etimberg I merged this one because you previously approved it (I just rebased since and addressed @benmccann comment).