csv-export plugin: filename options

added 2 options to the csv-export plugin, and edited plugins/README.md to document:
* initialFilename: sets the export filename for the download link (defaults to "Export.csv")
* editableFilename: boolean that controls whether to provide a text input for the user to set the export filename (defaults to false)

These changes are fully backward-compatible: the default values for the new options match the existing behavior.