Detect dynamic imports

[WIStudent]

While looking into this issue in precinct I figured out that it is node-ast-module-types that does not detect a script as an es6 module if it contains a dynamic import but no exports or other imports. I extended the isES6Import function to return true when it encounters a dynamic import. The definition for dynamic imports in ESTree can be found here https://github.com/estree/estree/blob/master/experimental/import-expression.md

[mrjoelkemp]

Thanks a ton for contributing @WIStudent. I just released this in a new minor version.