Allow expressions in the sortable attribute

[elnur]

I need to be able to choose a column to sort on dynamically like in the following example:

<td sortable="folder === 'inbox' ? 'from.username' : 'to.username'">

Currently, it's not possible because the sortable attribute accepts a literal instead of an expression.

If there's a workaround I could apply now, please tell me about it.

[elnur]

I've found that #82 implemented the feature but for some reason a newer commit overwrote it. Could you restore it back please?

[elnur]

Great. Thanks.