Merge pull request #1226 from larsbutler/geojson-export-prep

GeoJSON export prep

[r=matley,micheles,pslh] [f=*1198843]
bin/openquake:

Added '--export-type' command line option.

Options are 'xml' and 'geojson'.
bin/openquake:

Cleaned up documentation for `Export` command line options.
bin/openquake:

"store", don't "append" the --export-type parameter.
bin/openquake:

Pipe the --export-type parameter to the export functions.
export/risk:

Added loss_map geojson export support.