Add generic visual selection _extension_ using existing text objects

[Benoit de Chezelles]

Original issue: kana/vim-textobj-indent#10

Maybe it's just that I want to extend the visual selection, whereas usually text objects select some text but can't extend existing selection.

A better solution could be to add the extension capability to text objects, make it available for all of them? (using x (hear 'extend') in visual mode seems to not have downsides since d does same thing).
Could make v....xai to visual select some stuff, then eXtend selection with ai text object, based on current cursor position!