Add better interop for importing modules with default exports

[DylanPiercey]

Description

Currently when using the import tag to import es6 modules with default exports you instead get the entire exports object like { default: ..., }.

This change checks to see if the default export is set and tries to use it if possible like so:

import myThing from './myfile.js'

becomes

var module_myThing = require('./myfile.js')
var myThing = module_myThing.default || module_myThing

Motivation and Context

Resolves #802.
Currently users have to manually access the default export which is not in line with the modules spec.

Checklist:

• My code follows the code style of this project.
• I have updated/added documentation affected by my changes.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[coveralls]

Coverage increased (+0.002%) to 89.615% when pulling 472bf3b on DylanPiercey:import-default-interop into 76eaaf9 on marko-js:master.

[austinkelleher]

Nice changes. Looks good to me. I will let @mlrawlings and at @patrick-steele-idem review as well.