EPS / EMF Support

[jonmmease]

Orca has support for exporting EPS and EMF image formats, and we should do the same in Kaleido for feature parity.

• For EPS, this would involve performing PDF export to a temporary file and then using the pdftops command-line utility from poppler. This would introduce an optional dependency on poppler.
• For EMF, this would involve performing SVG export to a temporary file and then using inkscape from the command-line to convert the SVG file to and EMF file. This would introduce an optional dependency on inkscape.

Both poppler and inkscape are available as conda packages, so we could consider making these hard dependencies of the conda package.

[jonmmease]

Done in 01cf0df.

Kaleido should now have feature parity with Orca