[jss-extend][jss-compose] support array of rule names

It looks like a single string is accepted to pass an "existing" rule, but what if more than one has to be extended?

Example:
```
cellMinWidth: {
minWidth: 50,
},
blue: {
backgroundColor: 'blue',
},
serviceTable: {
'& th': {
verticalAlign: 'middle',
textAlign: 'center',
padding: 0,
extend: ['cellMinWidth', 'blue'], //not valid
},
'& td': {
verticalAlign: 'top',
textAlign: 'center',
padding: 0,
extend: ['cellMinWidth', 'blue'], //not valid
},
},
```

`jss-compose` doesn't seem to fit too:
```
cellMinWidth: {
minWidth: 50,
},
blue: {
backgroundColor: 'blue',
},
serviceTable: {
'& th': {
verticalAlign: 'middle',
textAlign: 'center',
padding: 0,
composes: ['$cellMinWidth', '$blue'], //neither is applied
},
'& td': {
verticalAlign: 'top',
textAlign: 'center',
padding: 0,
composes: ['$cellMinWidth', '$blue'], //neither is applied
},
},
```