rest_api_elections example could not find function "empty_election_df"

Hi grahamrp,

When I run:

`shiny::runApp(system.file("shiny-examples/rest_api_elections", package = "dtdatasources"))`

I get the error:

`Error in empty_election_df: could not find function "empty_election_df"`

I think you just need to export the `empty_election_df` function from 
[rest_api_election.R](https://github.com/grahamrp/dtdatasources/blob/master/R/rest_api_election.R)?
