Added multiple backend support into update_index command. Fixes #552
Merge remote-tracking branch 'torchbox/master' into searchchanges/updateindex-multi-backend
Added opton to update_index to allow user to select which backend to update
Docs for update_index --backend option
Pass backend name to update_index command in tests
Merge branch 'searchchanges/updateindex-multi-backend' of https://github.com/kaedroho/wagtail into kaedroho-searchchanges/updateindex-multi-backend
Fix option handling in update_index and refactor to avoid unnecessary backend/backend_name juggling
Release note for #556
Merge branch 'kaedroho-searchchanges/updateindex-multi-backend'