feat(validate): initial implementation

This is a bare-bones implementation of a basic validation package based
around a very simply Validatable interface:

    type Validatable interface {
        Validate() error
    }

The goal is to keep things as simple as possible, while also giving as
much control as possible over how validation logic is performed.

go.mod

@@ -0,0 +1,9 @@
+module github.com/romdo/go-validate
+
+go 1.15
+
+require (
+	github.com/stretchr/testify v1.7.0
+	go.uber.org/atomic v1.9.0 // indirect
+	go.uber.org/multierr v1.7.0
+)