SVG support when svglib available

[audoh-tickitto]

Addresses #73.

Adds support for SVGs if svglib is present.

Changes

• Added PmlImage.getDrawing() to return a ReportLabs Drawing
• Updated reportlab_paragraph._putFragLine to render image Drawing if available instead of the raster image
• Added PmlImage.getDrawingRaster() to rasterise a Drawing
  • Note: only used for backwards compatibility with getImage()
• Fixed tuple passed to setCharSpace when style.letterSpacing is (amount, unit)
• Fixed BytesIO passed pisaTempFile rather than bytes when pisaTempFile passed to PmlImage

[ulliholtgrave]

@luisza Hey,
Our project is really interested in this feature and we wanted to ask if we can help with anything to speed up the process? If developer hours are needed, feel free to contact us. We would also be willing to put an issue bounty on the feature if your project supports something like that :)
And thanks for your great work. xhtml2pdf is really super helpful! 👍