Support placeholder tokens in t.log()

[novemberborn]

t.log() is modeled after console.log(), which in Node.js uses util.format(). This function supports placeholder tokens. I couldn't quite find a maintained & ready module that would let us support the same tokens.

This is the behavior I'd expect:

Token	Behavior
%s	Cast the value to a string
%d	Cast the value to a number
%i	Parse the value as an integer
%f	Parse the value as a float
%j	Stringify the value as JSON, but output '[Circular]' if it contains circular references
%o	Stringify using Concordance (current behavior since #1653)
%O	Stringify using Concordance (current behavior since #1653)
%%	Print '%'

[sindresorhus]

Can't we just run it through util.format()?

[novemberborn]

That would make the formatting inconsistent with all our other value output.