Multiple entry points

I get this error:
```
W20190530-11:41:01.327(2)? (STDERR) TypeError: singleWebpackConfig.entry[key].push is not a function
W20190530-11:41:01.327(2)? (STDERR) at arrangeConfig (packages/ardatan_webpack-dev-middleware.js:242:48)
```

For this config:
```
let clientConfig = {
entry: {
main: './client/main.tsx',
widget: './client/widget.tsx'
},
...
}
```

Can be fixed by:
```
let clientConfig = {
entry: {
main: ['./client/main.tsx'],
widget: ['./client/widget.tsx']
},
...
}
```

I will create a PR to fix it.

Start at arrangeConfig in packages/ardatan_webpack-dev-middleware.js around line 242 and reproduce the error with the entry object shown in the issue. Verify that entries supplied as single paths are handled without the TypeError and that multiple named entry points still configure successfully.

Written by the indexing model from the issue text.