Add mechanism to easily get server side js string for debug

when debugging server side rendering if you encounter errors you need to see the files generated.

```
module MiniRacer
class Context
alias original_eval eval

def eval(str, options = nil)
original_eval str, options
rescue Exception => e
raise e
ensure
FileUtils.mkdir_p('./tmp') unless File.directory?('./tmp')
File.write('./tmp/prerendered-source.js', str)
end
end
end
```

Something like the above mini racer patch with the possibility of a config switch to set the file path. empty path means don't generate it.