Add option to set additional font metadata

[lunaroja]

Most font editors have ways of modifying other metadata. This would help when creating iconfonts that support different weights.

font-face font-family="Super Sans" font-weight="bold" font-style="normal"

https://developer.mozilla.org/en-US/docs/Web/SVG/Tutorial/SVG_fonts

options.fontWeight options.fontStyle

https://github.com/nfroidure/svgicons2svgfont/blob/master/src/index.js#L258

[nfroidure]

Would accept a PR with tests allowing such options.

[lunaroja]

Brilliant! Thank you!