More focus on a11y

[kazzkiq]

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:

• Not accessible at all #19
• How to close tooltip on second click or click away on mobile #60
• Tooltip on focus #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);