bugfix: slug in categoryByPage and categoryVar check in frontmatter

[avionics18]

@brob as mentioned in the README, categoryVar is the variable in the frontmatter that is used to assign categories to your content. And categoryCollection is used if you want to name the collection different from the value that is set in categoryVar.

Thus, this pull request fixes two things:

• categoryVar is used to check in the frontmatter of posts
• for posts having categories with spaces e.g: ['dev', 'UI Designs'], the slug wasn't properly genearted, added slugify.

.eleventy.js and articles in the demo/ folder have been updated to demonstrate this new change.

[olegfedak]

@avionics18, your changes have solved the issue #9 that I got not long ago.