Add filtering functionality to blog and portfolio

[ricardobalk]

• Added this to Milestone 3.4, due Aug 1, 2020.
• Time needed: 120 minutes +/- 50%.
• Already made a basic filter, shown below:

const filterPosts = ({ category }) => {
    Array.prototype.forEach.call(document.querySelectorAll('article[data-category]'), (e) => {
        e.style.display = (e.dataset.category == category ? '' : 'none');
    })
};

That will basically add style="display: none;" to each article[data-category] except the ones you'd like to see. e.g. filterPosts({ category: "front-end-development"}); will show article[data-category="front-end-development"] and hide others.

• This needs to be turned into an interactive Vue component.

[ricardobalk]

This feature has been implemented in dfdb12ef6e2b7b66f0003f5ac743ac189c796538. It needs refactoring, but a separate issue has been opened for that (#75).