add a way to import customConfig's.

Have an import method, like `client.customConfig.import(arg)`

arg can be an array or map, with the config intern
for example: 
```javascript
var data = {guildId: "1234", data: {prefix = ["!"]}}
client.customConfig.import(data)
```
or 
```javascript
var data  = new Map()
map.set("1234", {test: true})
client.customConfig.import(data)
```