Fix Flow error when using formatMessage.namespace
Fix Flow error when using formatMessage.namespace

When using `formatMessage.namespace`,
Flow would generate an error like this:

```
Cannot call formatMessage.namespace because function [1]
is incompatible with global object [2].

     src/some-code.js
      01│ import formatMessage from 'format-message'
      02│ const t = formatMessage.namespace()
      03│
      04│ t.setup({
      05│   formats: {

     node_modules/format-message/index.js
 [1]  54│   function formatMessage (msg/*: Message */, args/*:: ?: Object */, locales/*:: ?: Locales */) {
        :
 [2] 194│ module.exports = exports = namespace()
```

The solution I found was to explicitly type
what the `namespace` function returns
and attach the `namespace` function to `formatMessage`
within the `namespace` function.

thetalecrafter/format-message,format-message/format-message