Add support for multiplot options

[tomwhatmore]

I re-implemented the multiplot options from pull request #47 using @SiegeLord's feedback on the API and where to generate the gnuplot commands. I also added an example showing how to use the new options.

[SiegeLord]

Thanks! This looks better. I'll need to test things out more, but I'll start with this baseline. In particular, it's odd to me that some things are tied to the multi-layout which don't have to be (like title). I want to revisit whether we can always call set multiplot... it seems I disabled this in 2416ef8 for animation support, but I'm not completely sure why I did that.

[tomwhatmore]

I also felt like there was some weird over-coupling with the multi-layout logic, but I deliberately avoided getting into the larger questions of where and when the project calls set multiplot because it seemed out of scope for this feature, and additionally I didn't feel like I fully understood the existing logic. Just adding the extra options to the existing call seemed much safer!

Not sure how much I can help with any larger refactoring, but if I can make any changes to this PR that would make it easier let me know.

[SiegeLord]

A small update on this. It appears to not be a problem to just use set multiplot all the time. A bigger problem is the code right now calls reset before every axis, which nukes the multiplot options and breaks the multiplot_options example. Removing reset makes things like X-labels etc bleed across plots. Fixing #40 will remove the need to call reset, but that's a somewhat big effort.

[SiegeLord]

I've merged this with a number of supporting changes, thanks for the initial work!