Add useDynamicImport option, stable baseUrl

Split previously experimental into two stable options:

*   `useDynamicImport` — compile import statements into dynamic import
    expressions
*   `baseUrl` — resolve relative import specifiers from a given URL

This additionally fixes two bugs in the `'function-body'` output format:

*   `export * from 'a'` was not supported
*   `export {a as b}` was inverted

Closes GH-23.
Related-to GH-26.
Closes GH-30.