feat: support dynamicImport.loadingComponent as stateless function str

[sorrycc]

e.g.

export default {
  plugins: [
    ['umi-plugin-react', {
      dynamicImport: {
        loadingComponent: `() => <div>...</div>`,
      },
    }],
  ],
}

[coveralls]

Pull Request Test Coverage Report for Build 1768

• 1 of 5 (20.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.02%) to 33.274%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
packages/umi-plugin-react/src/plugins/dynamicImport.js	0	4	0.0%

Totals
Change from base Build 1766:	-0.02%
Covered Lines:	1054
Relevant Lines:	3249

---

💛 - Coveralls