Adding threshold option to decimation plugin

[Nico-DF]

Improvement proposition

Adding a new option in the decimation plugin to change the threshold used to detect if the decimation must be applied

Reason

Currently, to check if the decimation plugin must be called, Chart.js will check the number of point to display and decide to not do anything if the number of points is below 4*canvasWidth.

While in most of cases it is enough, in some cases I find that this threshold is a bit too high

In this case, the number of points is 1183 per dataset and the width is 800px hence the decimation is not called.
There might be other solutions than introducing this variable (such as modifying the scale, editing graph display, etc.) but I think it would be quite useful and is retro-compatible as it will fallback to the old 4*width.

options.plugins.decimation = {
    algorithm: 'lttb',
    enabled: true,
    samples: 200,
    threshold: 100
}

[etimberg]

Approved, but will hold off merging until a 3.4.1 goes out the door. We've got one bug fixed queued up there already