support simple dynamic paths in dynamic imports

Hi,

This is a follow-up to #216.

Currently transforming the following:
```ts
const getCommand = async (name: string): Command =>
(await import(`./commands/${name}.ts`)).default
```
yields the following result:
```js
const getCommand = async (name) =>
(await import(`./commands/${name}.ts`)).default
```

While I understand full support for dynamic paths is unsolvable, could the transformer support simple cases like this (either by inspecting the argument's AST or via a special annotation like webpack does) ?