Request for `igo run`

[tiye]

Since I have been using CoffeeScript for a long time, there are some practice that I think maybe some of them fit igo too.

• In CoffeeScript, running coffee enters interactive mode there's another igo for that. But for Go, repl might be less useful.
• When a user runs coffee file.coffee, code in that file is evaluated directly, just like it's just a language and steps of debugging does not contain something like "convert it to another language".
• Putting .js and .coffee file side by side is quite painful when the projects get larger. Because theres a lot of changes people click by mistake.

I suppose igo run would make it a lot simpler for people to use igo.