Add optional params to prototype spec

Fixes #69.

Currently we allow users to specify prototypes using Jsonnet. At the
head of each prototype should be a comment with semi-structured
metadata, e.g.,

  // @apiVersion 0.0.1
  // @name io.ksonnet.pkg.nginx-simple
  // @description Deploys a simple, stateless nginx server with server
  // blocks (roughly equivalent
  //   to nginx virtual hosts). The nginx container is deployed using a
  //   Kubernetes deployment, and is exposed to a network with a service.
  // @param namespace string default Namespace in which to put the
  // application
  // @param name string Name to give to each of the components

Critically, some metadata about params is stored using the `@param` tag,
including name, type, and description, for pretty printing.

This commit will introduce a new param type, `@optionalParam`. This type
has a name, type, default value, and description.

It is currently not possible to specify a default value with spaces, but
this should be good enough for...

prototype/specification.go

@@ -19,6 +19,7 @@ const (
 	nameTag        = "@name"
 	descriptionTag = "@description"
 	paramTag       = "@param"
+	optParamTag    = "@optionalParam"
 )
 
 func FromJsonnet(data string) (*SpecificationSchema, error) {
@@ -93,7 +94,7 @@ func FromJsonnet(data string) (*SpecificationSchema, error) {
 		openText = bytes.Buffer{}
 		openText.WriteString(strings.TrimSpace(split[1]))
 		switch split[0] {
-		case apiVersionTag, nameTag, descriptionTag, paramTag: // Do nothing.
+		case apiVersionTag, nameTag, descriptionTag, paramTag, optParamTag: // Do nothing.
 		default:
 			return nil, fmt.Errorf(`Line in prototype heading comment is formatted incorrectly; '%s' is not
 recognized as a tag. Only tags can begin lines, and text that is wrapped must
@@ -222,6 +223,27 @@ func (s *SpecificationSchema) addField(tag, text string) error {
 			Default:     nil,
 			Type:        pt,
 		})
+	case optParamTag:
+		// NOTE: There is usually more than one `@optionalParam`, so we
+		// don't check length here.
+
+		split := strings.SplitN(text, " ", 4)
+		if len(split) < 4 {
+			return fmt.Errorf("Optional param fields must have '<name> <type> <default-val> <description> (<default-val> currently cannot contain spaces), but got:\n%s", text)
+		}
+
+		pt, err := parseParamType(split[1])
+		if err != nil {
+			return err
+		}
+
+		s.Params = append(s.Params, &ParamSchema{
+			Name:        split[0],
+			Alias:       &split[0],
+			Description: split[2],
+			Default:     &split[3],
+			Type:        pt,
+		})
 	default:
 		return fmt.Errorf(`Line in prototype heading comment is formatted incorrectly; '%s' is not
 recognized as a tag. Only tags can begin lines, and text that is wrapped must