Add support for requestInterceptor / responseInterceptor.

- Display mutated requests from request interceptor in curl output in UI.
  Put this behind showMutatedRequest flag so that the mutation can be
  silent.
- Document requestInterceptor, responseInterceptor and showMutatedRequest
  in README.md
- Add tests

src/core/plugins/spec/selectors.js

@@ -237,6 +237,11 @@ export const requests = createSelector(
     state => state.get( "requests", Map() )
 )
 
+export const mutatedRequests = createSelector(
+    state,
+    state => state.get( "mutatedRequests", Map() )
+)
+
 export const responseFor = (state, path, method) => {
   return responses(state).getIn([path, method], null)
 }
@@ -245,6 +250,10 @@ export const requestFor = (state, path, method) => {
   return requests(state).getIn([path, method], null)
 }
 
+export const mutatedRequestFor = (state, path, method) => {
+  return mutatedRequests(state).getIn([path, method], null)
+}
+
 export const allowTryItOutFor = () => {
   // This is just a hook for now.
   return true