feat: method to download files easily

[pierresh]

This PR adds the method download() to download easily a file from the server.

Example:

$file = 'path/to/file.txt';

Flight::download($path);

[n0nag0n]

Thanks for creating this PR. When I get around to it, I'll have to change a bit of this code to conform to the contribution requirements (unit testing, syntax requirements, etc). For now I assume you're good with just keeping this as a Flight::map('download', ...); method in your project correct?

[pierresh]

Yes, I can use map() for time being, thanks!

[n0nag0n]

Wanna look this over in case I missed anything?

[pierresh]

I checked the code and ran the test, it looks good to me, thanks!