Allow customization of columns in list view #195

Currently, the list view is hardcoded to show columns for Artist and Series/Parody.
A good improvement would be to allow the user to specify which namespace he wants to be shown in a column. (Or if he wants none, to hide said columns.) This would allow for customizable sorting on specific namespaces.

Implementing this would start in index_datatables.js, by modifying the three var columnX variables to match namespaces entered by the user and stored in localStorage. Each variable matches the namespace that'll be displayed in the matching column.