More focus on a11y

[Claudio Holanda Jr.]

This Issue will lead to breaking changes.

As already discussed in other issues, Balloon.css should be using more acessible attributes in order to show tooltips.

Those Issues are going to be centered here:

• #19
• #60
• #86

This issue aims to solve:

• Use aria-label for tooltip texts;
• Enable tooltip on :focus state (but also add an option to disable focus);