add proxyTable support

[chimurai]

Suggested in #4 (comment)

proxyTable option allows you to use a different option.target based on host request header and optional path.

Proposal:

proxyMiddleware('/api', {
  target: 'http://www.example.org',
  proxyTable : {
    'foo.com/api': 'http://website.com:8000',
    'foo.com': 'http://website.com:8001',
    'bar.com': 'http://website2.com:8002'
  }
});