Add a header with total of selected columns

[philippemilink]

Hi, I added an option to compute the total of a column and display it in a row just under the header row.

Requires PR #836, #816, #837.

[stephanvierkant]

@philippemilink Can you resolve the conflicts? I'd like to give it a try :)

[philippemilink]

I just solved the conflicts, all needed PRs were merged.

I tested it with https://github.com/stwe/DtBundleDemo10, just add 'compute_total' => true in src/AppBundle/Datatables/PostDatatable.php:222. It will display the total of ratings of all posts; totally meaningless, but it is a total of integers ! 😄

While testing with demo app, I discovered a bug, fixed with PR #916.

[stephanvierkant]

Thanks! I will look into this asap!