improve Table API with regards to Sorting

[mbohal]

Presently it is possible to express in Table component API invalid configuration.

It is valid to set columns.isSortable = true and not provide the sort object. We could validate this via a custom Prop validation function, but I think it more elegant to:

• remove the columns.isSortable param
• add new sort.sortableColumns param holding an array of column names

With this change the user only has to ensure that the column names within params columns, rows, and sort match.