DataTable Column custom sorter support

[gatapia]

It would be good if columns supported functions in the sortable argument. I.e.

<p-column field="x" [sortable]="customsorter">...

Where custom sorter is a standard sorting function , i.e:

customsorter(column: string, row_a: any, row_b: any): number
// i.e. return -1, 0, 1 defending on comparison

Or perhaps as a temporary hack [sortable] could take a string pointing to another column.

This feature ofcourse would allow smart sorting on ordinal fields, such as "Severity" where alphabetical order makes no sense.

[cagataycivici]

Sounds good. Maybe we can add sortFunction;

<p-column sortable="custom" (sortFunction)="customSort($event)"

Similar to #196