Allow optional params (#68)

* Allow optional params

* Don't double-up on method signature

* Cleanup formatters/util

packages/api-jsonrpc/src/state/getStorage.js

@@ -19,6 +19,7 @@ module.exports = ({
       type: 'H256'
     },
     {
+      isOptional: true,
       name: 'blockHash',
       type: 'HeaderHash'
     }

packages/api-jsonrpc/src/types.js

@@ -10,6 +10,7 @@ export type FormatInputType = 'Bytes' | 'H256' | 'HeaderHash' | 'String';
 export type FormatOutputType = 'BlockNumber' | 'Bytes' | 'Header' | 'HeaderHash' | 'U64';
 
 export type InterfaceInputType = {
+  isOptional?: boolean,
   name: string,
   type: FormatInputType
 };