Add NumbersAsStrings to EncoderConfig

[fishy]

This partially fixes uber-go#884.

This implementation is limited to top level fields, like:

Int64("int64", 123)
Float64("float64", 456)

Or sugared:

With("int64", 123, "float64", float64(456), "slice", []int{1, 2, 3})

It does NOT work with reflected fields like:

Reflect("slice", []int{1, 2, 3})
With("struct", struct{ Int64 int64 }{Int64: 123})

As for reflected fields we use go's stdlib json encoder directly, which
doesn't provide such feature.