fread_folder is broken w.r.t. recent data.table

Hello, in our efforts to prepare `data.table` for an update on CRAN (-> v1.11.0), we noticed that recent changes will break your package:

```
install.packages('data.table', type = 'source', repos = 'http://Rdatatable.github.io/data.table')
pkg = 'easycsv'
install.packages(pkg, dependencies = TRUE)

tools::testInstalledPackage(pkg, outDir = "~/Desktop/dt_revdep")

library(pkg, character.only = TRUE)
fn = 'fread_folder'
example(fn, character.only = TRUE)
```

> Error: isTrueFalse(showProgress) is not TRUE

This is because the default argument of `showProgress` has changed to be equivalent to the output of `interactive()`, and the default value of `getOption('datatable.showprogress')` has been deleted.

Please follow https://github.com/Rdatatable/data.table/pull/2785 and feel free to provide input there if you have strong opinions about how to proceed.

Note that this issue also effects your `SIRItoGTFS` package; I'll file an official issue there once the aforementioned pull request is resolved, if needed.