allow filtering of tables lazy loaded using dbplyr

hey!
I just wanted wanted to create this issue so I can refer to it in my [pull request](https://github.com/dgkf/shinyDataFilter/pull/27)

Basically, I think this is a fantastic module, but my main shiny usage is with databases, so it wasnt compatible.
I created a pull request with code that "works".
However:
* I had to give up on the "filtering at every step" as you add filters. instead the filtering only occurs at the end when displaying the resulting table.
* character column are treated as factors (because there are no factors in databases)
* The full vector is being downloaded from the database when creating the filters. It would be much better to download the counts of each possible values. (better to get the count of male and female instead of getting a 6-million long character vector).