Support filter option for full control over path matching

[atesgoral]

https://github.com/chimurai/http-proxy-middleware#http-proxy-middleware-options

custom matching

For full control you can provide a custom function to determine which requests should be proxied or not.

/**
 * @return {Boolean}
 */
var filter = function (pathname, req) {
    return (pathname.match('^/api') && req.method === 'GET');
};

var apiProxy = proxy(filter, {target: 'http://www.example.org'})

With this small change, it is possible to pass a filter property in a proxy table entry to use a request filter function instead of using the proxy table key as the context. The context is ignored. For example:

proxyTable: {
  '/': { // The root page is rendered the backend
    target: 'http://localhost:6543',
    filter: (pathname, req) => {
      return pathname === '/'; // Only proxy the root path, bypass for other build artifacts
    }
  },
  '/api': {
    target: 'http://localhost:6543'
  },
  ...