End to bridge encryption implementation

So far this is passing my basic tests, but could use some testing from people
that are much more familiar with how this is supposed to work.

Refs #27

config/encryption.go

@@ -0,0 +1,29 @@
+package config
+
+type encryption struct {
+	Allow   bool `yaml:"allow"`
+	Default bool `yaml:"default"`
+
+	KeySharing struct {
+		Allow               bool `yaml:"allow"`
+		RequireCrossSigning bool `yaml:"require_cross_signing"`
+		RequireVerification bool `yaml:"require_verification"`
+	} `yaml:"key_sharing"`
+}
+
+func (e *encryption) validate() error {
+	return nil
+}
+
+func (e *encryption) UnmarshalYAML(unmarshal func(interface{}) error) error {
+	type rawEncryption encryption
+
+	raw := rawEncryption{}
+	if err := unmarshal(&raw); err != nil {
+		return err
+	}
+
+	*e = encryption(raw)
+
+	return e.validate()
+}