Add to_file task to export namespace
Add to_file task to export namespace

This implements the build function on the new TaxonTreeExport
class to build the taxon tree array and convert it
to JSON. Then saves the JSON to a file in the temp directory

The json task now also implements the TaxonTreeExport build
function.  
alphagov/content-tagger,alphagov/content-tagger,alphagov/content-tagger