feat(tooltip): add mdTooltipClass

[willshowell]

Fixes #4601

• I wasn't able to identify a reason for ViewEncapsulation to be on for the tooltip, so I set it to none. Will gladly revert if this is incorrect, but it will help users with their css customizations.
• I chose to support the ngClass syntax to match select's panelClass. But if the user uses the key/value format and adds or removes a class that affects tooltip size while it's showing, its position won't update.

[andrewseguin]

LGTM, thanks for adding this

[angular-automatic-lock-bot]

This issue has been automatically locked due to inactivity.
Please file a new issue if you are encountering a similar or related problem.

Read more about our automatic conversation locking policy.

_{This action has been performed automatically by a bot.}