adding global_root options

Adding global_root parameter. The main motivation is to have global variables in the root namespace "gon" instead of "gon.global". That way user can set global values and override with local values.

Examples:

```
    = include_gon
or
    = include_gon(global_root: 'global')
```

Keeps the default behavior using "gon.global" for global variables. 

```
    = include_gon(global_root: 'g')
```

The same as the default behavior, only the global variable will end up in 'gon.g'.

```
    = include_gon(global_root: nil)
or
    = include_gon(global_root: '')
```

Use "gon" for global variables. If there's a locally defined variable, that will override the global one.