Initial support for default imports/exports, see #98

Does not implement combinations like 'import theDefault, *' yet

tests/parser/export-default.ts.fixture.ts

@@ -0,0 +1,7 @@
+export default function theFunction(): void {}
+export default class TheClass {}
+export default enum TheEnum {}
+export default namespace theNamespace {}
+export {
+  something as default
+};