add {model.field} support

[gaydenko]

It would be handy to have an opportunity to select opening and closing strings for interpolation. Say, not-doubled curly braces can be preferred by someone as me :)

[gaydenko]

In accordance with knockout/knockout#1273 there is a plan to use interpolation for attributes values also, not just for text. So, single curly braces would interference with bindings options. As a result and for the sake of simplicity I'm closing the issue :)

[mbest]

I'm reopening this because it's something I'd like to look into supporting.