Add back a way to use default formatters

[benmccann]

From @adamk33n3r - #4602 (review) removes the possibility to use the default formatters via Chart.Ticks like

userCallback: (value, index, values) -> Chart.Ticks.formatters.linear(value, index, values) + '%'

It turned out that I ended up having the same issue in the financial chart as well. I had to copy the Ticks.formatters.linear implementation there:
chartjs/chartjs-chart-financial@0d47e06#diff-7499900c079d3c33e145ddba2be7d916

[benmccann]

From @cholt0425:

I also created an extension and I used the default linear formatter for the linear gauge I created and now I can't access it to use it as the default callback.

[simonbrunel]

Fixed by #5088