"Script mode" tweaks

[clemos]

• fix error reporting (offsets are quite obviously wrong)
• implement custom class name (?)

[markknol]

if you start with code comment, the complete class doesnt compile
https://try.haxe.org/#A9765

// Is haxe great?
class Test {
    static function main() {
        trace("Haxe is great!");
    }
}

Build failure
Test.hx:8: characters 0-5 : Unexpected class

[tariqbenezza]

I'm on it

[clemos]

We probably want to trim more than whitespaces here : https://github.com/clemos/try-haxe/blob/master/src/api/Compiler.hx#L94
We can use something like this instead of ltrim : https://try.haxe.org/#467a7