Document vertical tabs input properties

[blackbaud-johnly]

#1798. Document tabId, ariaControls, ariaInvalid, ariaRequired, and ariaRole properties.

New inputs on the SkyVerticalTabComponent:
tabId - An identifier for a vertical tab. This can be set if you need a primary identifier for your tab.
ariaControls - Should be set to the id of the vertical tab content pane that this tab controls.
ariaRole - Should be set to the aria-role of the tab. Defaults to "tab".
ariaInvalid - If your tabset is part of a form, this can be used to show that the values in this tab's content pane are invalid.
ariaRequired - If your tabset is part of a form, this can be used to show that the values in this tab are required for the form to be complete.

[blackbaud-johnly]

Addressed in blackbaud/skyux2-docs#227.