Support shorthand proxy configuration

[chimurai]

Support shorthand proxy configuration.

Set target = http://www.example.org and proxy everything:

var proxy = proxyMiddleware('http://www.example.org');
// equals:  proxyMiddleware('/', {target: 'http://www.example.org'});

Set target = http://www.example.org and context = '/api':

var proxy = proxyMiddleware('http://www.example.org/api');
// equals:  proxyMiddleware('/api', {target: 'http://www.example.org'});

[chimurai]

Usage with wildcard:

proxyMiddleware('http://www.example.org:8000/api/books/*/**.json');
// proxyMiddleware('/api/books/*/**.json', {target: 'http://www.example.org:8000'});

Usage with options configuration

proxyMiddleware('http://www.example.org:8000/api', {changeOrigin:true});
// proxyMiddleware('/api', {target: 'http://www.example.org:8000', changeOrigin: true});