Auto-completion for attributes + built-in attributes doc. fix #243

[bbrenne]

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing documents:

The PR fulfills these requirements:

• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)
• All tests are passing: https://github.com/asciidoctor/asciidoctor-intellij-plugin/blob/master/CONTRIBUTING.adoc#getting-started-with-coding
• New/updated tests are included

If adding a new feature:

• Feature documentation is updated: https://github.com/asciidoctor/asciidoctor-intellij-plugin/blob/master/FEATURES.adoc

Other information:

[bbrenne]

Known bug: some attribute description are bugged due to the presence of html tags in them, I did'nt find a way to escape them yet.

[ahus1]

Looks great! I am testing it for some corner cases and I started to prepare a commit to fix the HTML escaping. I will merge it over the next one or two days.

[ahus1]

I've squashed your commits with the code style and test fixes. I've added:

• selecting an entry from autocomplete for a built-in attribute that is replacing an attribute with a dash (will replace complete existing attribute instead of the first part)
• adding a ":" or a "}" only when there is not there already
• escaping of HTML
• documentation is now also available on attributes and attribute references in the editor (not only during autocomplete)
• a PSI unit test
• updated documentation