added symbolPrefix

[epikhighs]

allowing a prefix to the symbol IDs was an option in the other SVG libs I used. So I thought I'd add one here.

[Climenty]

@epikhighs, what about using the same CSS prefix (svg_) for symbol naming? Or does your use case require a different option?

[epikhighs]

I thought about that, but there's a default value for css prefix that seemed to be applied even if i don't specify the css prefix option. So then, that would change the behavior to warrant a major version bump right? This solution would keep backwards compatibility.

but yeah, I would think that most people would use the same prefix for css and svg Ids