Support formatting specifiers

[holtwick]

https://console.spec.whatwg.org/#formatting-specifiers

Consider log(fmt("Test %o", obj)) to make it work with console hack.

Specifier	Purpose	Type Conversion
%s	Element which substitutes is converted to a string	%String%(element)
%d or %i	Element which substitutes is converted to an integer	%parseInt%(element, 10)
%f	Element which substitutes is converted to a float	%parseFloat%(element, 10)
%o	Element is displayed with optimally useful formatting	n/a
%O	Element is displayed with generic JavaScript object formatting	n/a
%c	Applies provided CSS	n/a