allow font cleaning and remapping for SVG output

[nsonnad]

Hello. We are using saveSvgAsPng to make charts and export them as SVG, so that they can be hand-edited in Illustrator. There is a terrible issue with some versions of Illustrator where it gives a cryptic error if you have a font declaration with either 1) more than one font or 2) a font that is not on your system or named incorrectly.

This change adds two options to the library:

1. cleanFontDefs, a boolean that if true will extract only the first font-family definition
2. fontFamilyRemap, an optional hash that remaps the font-family name of the extracted first definition.

I have added an example to index.html that downloads an SVG declared as Georgia, but when you open in illustrator, it is set to Arial. The use case for this is when CSS fonts are declared using CSS conventions like Khula-Light while local fonts may show up as Khula Light, or when you can't assume people will have a custom font installed locally so you may want to remap it to Arial or something.

Let me know what you think, if you're good with this I can add docs on these options. Thanks

[exupero]

This is a very specific need. To accomplish the fix you need, I would rather provide a general hook that allows modifying the CSS that gets added for each rule, somewhat like selectorRemap but for the entire rule. Then you could apply this rule transformation within your own application and this library could support other specific needs along the same lines.