Add defaultScope to global config options

As requested [here] (https://github.com/fnando/i18n-js/issues/232) it will add defaultScope global setting that would allow skipping providing the same scope value over and over again. So instead of doing 

```
I18n.t("name", {scope: "activerecord.attributes.user"});
I18n.t("email", {scope: "activerecord.attributes.user"});
```
or
```
var defaults = {scope: "activerecord.attributes.user"};
I18n.t("name", defaults);
I18n.t("email", defaults);
```

we could simply:
```
I18n.defaultScope = "activerecord.attributes.user";
I18n.t("name");
I18n.t("email");
```
What do you think?