Better control of table styling, better styling of filter input

On my fork I've made two additions:

1. `reactive_table.html` has table styling hard coded, not really neat. For this reason I introduced the `class` argument to the template in order to explicitly specify which class to apply.
2. Put the filter input control inside a form-group, to have a better styling (some space between the input and the table).

Even if I think `reactive-table` need a more general overhaul regarding styling and theming, these two small modifications resolve some problems with a very little effort.
If you agree I'll be glad to have this pull request merged.