sequence: use simple theme

[LukeCarrier]

The sequence diagram type is not styled consistently with the diagrams. Using the simple theme included in js-sequence-diagrams addresses this.

[seflless]

Thanks again. I haven't continued development on this for a while. I totally want to. Thanks for the improvements. Glad to see someone using it.

[LukeCarrier]

@francoislaberge the thanks are all mine -- thanks for creating and releasing such a useful tool, and for such prompt merges. I'm using this tool for a bunch of documentation work, so I'll gladly contribute any fixes I make. :)

[seflless]

Awesome, yeah my use case was documentation and easy generation of explanatory diagrams. I was hoping to make it to auto generate the diagrams on save, when editing the textual presentation in Atom (and other editors). I didn't get too far in doing that. But here's an Atom Package that was the start of that:
https://atom.io/packages/diagrams