More accurate typescript typings

According to the API, all but the `src` prop are optional, but the `Props` interface had no optional properties, making anyone who uses typescript **have** to pass everything (or it would throw a type error and will not compile).
I updated the interface to make the denote the non-required props as optional :)

Author: illBeRoy

src/index.tsx

@@ -10,14 +10,14 @@ export type OnInjected = (
 ) => void
 
 interface Props {
-  evalScripts: 'always' | 'once' | 'never'
-  fallback: React.ReactType
-  loading: React.ReactType
-  onInjected: OnInjected
-  renumerateIRIElements: boolean
+  evalScripts?: 'always' | 'once' | 'never'
+  fallback?: React.ReactType
+  loading?: React.ReactType
+  onInjected?: OnInjected
+  renumerateIRIElements?: boolean
   src: string
-  svgClassName: string
-  svgStyle: React.CSSProperties
+  svgClassName?: string
+  svgStyle?: React.CSSProperties
 }
 
 interface State {