Add imports to guesser output

[fzaninotto]

Problem

Guesser output can't be copy/pasted directly, as it misses the import part.

Solution

Add the imports

Example output

Guessed Show:
+
+import { DateField, ReferenceField, Show, SimpleShowLayout, TextField } from 'react-admin';

export const CommentShow = () => (
    <Show>
        <SimpleShowLayout>
            <TextField source="id" />
            <DateField source="author.undefined" />
            <ReferenceField source="post_id" reference="posts"><TextField source="id" /></ReferenceField>
            <TextField source="body" />
            <DateField source="created_at" />
        </SimpleShowLayout>
    </Show>
);

[djhi]

There's a linter warning left, otherwise 👍