extend maxBuffer size

I got unexpected error.

```
Previewing ApiBlueprint Failed

stdout maxBuffer exceeded.
```

my environment.

```
$ ls -la apiary.apib
-rw-r--r--  1 vvakame  staff  121066 12  3 12:05 apiary.apib
$ aglio -i apiary.apib -o tmp.html
$ ls -la tmp.html
-rw-r--r--  1 vvakame  staff  402275 12  3 15:05 tmp.html
```

http://nodejs.org/api/child_process.html#child_process_child_process_exec_command_options_callback
> maxBuffer Number (Default: 200*1024)

I want resolve it.