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.
When `barThickness: undefined|null` (default), we compute an optimal sample size based on the smallest tick interval reduced to prevent any bar to overlap (bar equally sized). Also added support for a special `barThickness: 'flex'` value (previous default) that globally arranges bars side by side to prevent any gap when percentage options are 1 (variable bar sizes).
