Added Option for drawing chart lines dashed

[JCarstens]

I added methods and variables to allow generic dashing of lines in the framework.

A simple method for trying this out is adding dash styles to a line plot (I think my changes only work with them, haven't investigated other plots further) like this:

plot.replace_dash_styles([[5, 5],[9999]]);

The first dash style means 5 pixels line then 5 pixels space and so on, the second one is a solid line. In general, dashing styles work pretty similar to colors. If you want the dashing shown in the legend, you have to set the new line_legend option to true.

Please write me if you have any further questions and thanks for creating this great framework in the first place. :)