issue#2609 | Sasha | predictable axios requests

- axios requests are not delayed by pre-emptive promise creation by default
- add options to interceptors api ("synchronous" and "runWhen"
- add documentation and unit tests

README.md

@@ -530,6 +530,33 @@ const instance = axios.create();
 instance.interceptors.request.use(function () {/*...*/});
 ```
 
+When you add request interceptors, they are presumed to be asynchronous by default. This can cause a delay
+in the execution of your axios request when the main thread is blocked (a promise is created under the hood for 
+the interceptor and your request gets put on the bottom of the call stack). If your request interceptors are synchronous you can add a flag
+to the options object that will tell axios to run the code synchronously and avoid any delays in request execution.
+
+```js
+axios.interceptors.request.use(function (config) {
+  config.headers.test = 'I am only a header!';
+  return config;
+}, null, { synchronous: true });
+```
+
+If you want to execute a particular interceptor based on a runtime check, 
+you can add a `runWhen` function to the options object. The function will be called with the config
+object (don't forget that you can bind your own arguments to it as well.) This can be handy when you have an
+asynchronous request interceptor that only needs to run at certain times.
+
+```js
+    function onGetCall(config) {
+      return config.method === 'get';
+    }
+    axios.interceptors.request.use(function (config) {
+      config.headers.test = 'special get headers';
+      return config;
+    }, null, { runWhen: onGetCall });
+```
+
 ## Handling Errors
 
 ```js