Add enum field to boolean types (#71)

I'm using this library with the stripe API, and they've got a few places
where they use the enum field on a boolean to indicate that a field
will always be either true or false.

This PR updates the types in this crate to support that.  It's a small
change, but still probably counts as a breaking change, so not sure how
you want to handle that?

src/schema.rs

@@ -68,7 +68,7 @@ pub enum Type {
     Integer(IntegerType),
     Object(ObjectType),
     Array(ArrayType),
-    Boolean {},
+    Boolean(BooleanType),
 }
 
 #[derive(Debug, Clone, Serialize, Deserialize, PartialEq)]
@@ -214,6 +214,13 @@ pub struct ArrayType {
     pub unique_items: bool,
 }
 
+#[derive(Debug, Clone, Serialize, Deserialize, Default, PartialEq)]
+#[serde(rename_all = "camelCase")]
+pub struct BooleanType {
+    #[serde(rename = "enum", default, skip_serializing_if = "Vec::is_empty")]
+    pub enumeration: Vec<Option<bool>>,
+}
+
 #[derive(Debug, Clone, Serialize, Deserialize, PartialEq)]
 #[serde(rename_all = "lowercase")]
 pub enum NumberFormat {