To be able to evaluate options passed to DataTables

This is an attempt to solve an FAQ about datatables: users are unable to pass JS functions from R. I added a new argument `evalOptions` that takes a vector of names, and evaluate these elements in the options.

An example:

```s
runApp(list(
  ui = fluidPage(dataTableOutput('foo')),
  server = function(input, output) {
    output$foo = renderDataTable(
      iris,
      options = list(fnRowCallback = '
        (function(nRow, aData, iDisplayIndex, iDisplayIndexFull) {
          // Bold cells for those >= 5 in the first column
          if (parseFloat(aData[0]) >= 5.0)
            $("td:eq(0)", nRow).css("font-weight", "bold");
        })'
      ),
      evalOptions = 'fnRowCallback'
    )
  }
))
```

Thoughts? @jcheng5