feat: enable onload callbacks (#414)

* refactor(examples): run ssr example from server

* chore: switch to babel for build

buble complains too much

* feat: enable loaded callbacks

feat: add skip option

* examples: add async-callback browser example

* examples: fix server

* examples(ssr): add reactive script with callback

* fix: also skip on ssr

* chore: remove unused var

* feat: only add mutationobserver if DOM is still loading

feat: disconnect mutation observer once DOM has loaded

* examples: pass vmid to loadCallback instead of el

* feat: also support load callbacks for link/style tags

* test: add unit tests for load

* test: add load e2e test

* chore: fix lint

* chore: remove unused files

* test: fix e2e load callback test

* test: fix attempt

* examples: ie9 compatiblity

destructuring doesnt work in ie9

* fix: add onload attribute on ssr

dont rely on mutationobserver

* chore: lint ci conf

* refactor: remove loadCallbackAttribute config option

test: fix coverage for load

* test: improve coverage

* fix: only use console when it exists (for ie9)

* chore: fix coverage

test/fixtures/basic/static/load-test.js

@@ -0,0 +1 @@
+window.loadTest = 'loaded'

test/fixtures/basic/views/home.vue

@@ -24,7 +24,8 @@ export default {
       ],
       script: [
         { vmid: 'ldjson', innerHTML: '{ "@context": "http://www.schema.org", "@type": "Organization" }', type: 'application/ld+json' },
-        { innerHTML: '{ "more": "data" }', type: 'application/ld+json' }
+        { innerHTML: '{ "more": "data" }', type: 'application/ld+json' },
+        { vmid: 'loadtest', src: '/load-test.js', body: true, async: true, callback: () => (window.loadCallback = 'yes') }
       ],
       noscript: [
         { innerHTML: '{ "pbody": "yes" }', pbody: true, type: 'application/ld+json' },