Playground: More user friendly scheme for accessing raw snippet.

[dmitshur]

@dominikh made a valid observation that the GopherJS playground is currently missing a certain feature of the Go playground. The ability to add a ".go" suffix to the playground URL and get a raw snippet. This is useful to be able to curl the snippet to a remote machine, for example.

Compare, Go playground URLs:

https://play.golang.org/p/v3rrZLwEUC
https://play.golang.org/p/v3rrZLwEUC.go

GopherJS playground URLs:

http://www.gopherjs.org/playground/#/hy-CTQOoiM
http://www.gopherjs.org/playground/#/hy-CTQOoiM.go - does not work
http://snippets.gopherjs.org/p/hy-CTQOoiM - works, but is not user friendly. also doesn't have .go suffix, so requires the filename to be changed when downloading.

Thanks @dominikh for reporting.