Make line widths customizable

Hi!

I extended the Morris.Line class to support customization of the line widths when several , in a similar way of point widths and line colors. The use case originating this PR is to be able to draw boundary lines above and under the main depicted line. This will work in a similar way of goals do, but not only horizontal. Example: percentile of growth over age graphs for children.

For the sake of backwards compatibility, I've not changed the attribute name ``lineWidth`` for the plural form ``lineWidth``. I also allow both an Array of sizes and a single size to be provided. Is this a good approach?

I hope this is simple enough to be considered for Morris' codebase.

Cheers!