Abstraction for Certificate Storage (+ vault storage) (#406)

* new abstraction around storage

* re-work completely to fit new acme package pre-solving paradigm

* vault storage plugin

* add some vendor

* delete old vendor pinning mechanism

vendor/github.com/mitchellh/mapstructure/error.go

@@ -0,0 +1,50 @@
+package mapstructure
+
+import (
+	"errors"
+	"fmt"
+	"sort"
+	"strings"
+)
+
+// Error implements the error interface and can represents multiple
+// errors that occur in the course of a single decode.
+type Error struct {
+	Errors []string
+}
+
+func (e *Error) Error() string {
+	points := make([]string, len(e.Errors))
+	for i, err := range e.Errors {
+		points[i] = fmt.Sprintf("* %s", err)
+	}
+
+	sort.Strings(points)
+	return fmt.Sprintf(
+		"%d error(s) decoding:\n\n%s",
+		len(e.Errors), strings.Join(points, "\n"))
+}
+
+// WrappedErrors implements the errwrap.Wrapper interface to make this
+// return value more useful with the errwrap and go-multierror libraries.
+func (e *Error) WrappedErrors() []error {
+	if e == nil {
+		return nil
+	}
+
+	result := make([]error, len(e.Errors))
+	for i, e := range e.Errors {
+		result[i] = errors.New(e)
+	}
+
+	return result
+}
+
+func appendErrors(errors []string, err error) []string {
+	switch e := err.(type) {
+	case *Error:
+		return append(errors, e.Errors...)
+	default:
+		return append(errors, e.Error())
+	}
+}