add execAsStream execution option

[jeromew]

Here is a PR that adds the execAsStream feature as per #38

I don't really see how to test it programmatically since we would need a stable ftp server.

I use it

• with the find command to get a deep ls of a ftp server root
• with the cat command to stream-pipe the content of a remote binary file

Up to now it has behaved as expected (both for the success case and the error case triggering and 'error' event)

var stream = (new lftp(config)).raw('find').execAsStream(); stream.pipe(process.stdout);

[atinux]

Thank you a lot @jeromew 👏

It's now available on npm with the version 0.6.0