Implement support for DBGp Proxy servers - #237

As documented here:
https://xdebug.org/docs-dbgp.php#just-in-time-debugging-and-debugger-proxies

Defines two new config parameters:

• proxy_host (default '')
• proxy_port (default 9001)

If both options are non-empty, the Connection class does a few new things at the beginning and end of our session.

New workflow when using a proxy is:

• VIM listens for debugger connections on local IP and port X (default=9000) like always
• VIM makes a connection to the proxy server on port Y (default=9001)
• VIM sends a "proxyinit" command, which includes ide_key and the port we are listening on (X)
• Proxy registers our IP/port with our ide_key and sends an XML response
• VIM processes the XML response and disconnects
• VIM starts the usual 20s wait for debugger connection
• debuggers are all hard-coded to connect to the proxy for debugging sessions
• If a debugger connection comes into the proxy (before the VIM timeout) with our ide_key, the proxy will connect back to us on port X and copy all data back and forth between VIM and debugger. This connection is indistinguishable from a regular debugger connection (except originating from the proxy IP instead of the remote host IP).
• When we are done, VIM makes another connection to proxy port Y and sends a "proxystop" command to un-map our ide_key