Allow overriding the used transport

Update AxiosTransformer type in index.d.ts

Add request field to AxiosResponse type definition

Update TypeScript typings with generic type parameters

Adding axios-extensions to the ecosystem

Use XMLHttpRequest in IE10+ instead of XDomainRequest

Add bundlesize to test

Tiny fix

Update ECOSYSTEM.md

This allows users of axios inside `electron` to provide the [`net`](https://electron.atom.io/docs/api/net/) module as the http transport instead of using nodes http/https modules.  This gives a whole bunch of things to Electron users including automatic proxy resolution.

add axios-cache-plugin to ECOSYSTEM.md

* Adding a way to disable all proxy processing

When the proxy field in configuration is === false all proxy processing is
disabled. This specifically disable the 'http_proxy' environment variable
handling.

Fixes #635
Related to #434

* Change readme wording

From review comment on PR (#691)

* Update parseHeaders to match node http behavior

Node ignores duplicate entries for certain HTTP headers.

It also always converts the `set-cookie` header into an array.

* add tests for new duplicate header handling

* clarify comment

* Fixing baseURL not working in interceptors

* add test for  modify base URL in request interceptor

Convert the method parameter to lowercase

No buffer in browser