Add support for default tooltip option & tooltip functions

[techfg]

Is your feature request related to a problem and/or adding functionality? Please describe.
Currently, in order to trigger a tooltip, an area configuration must be supplied based on the mapKey and the tooltip only supports text and jQuery values. Having to specify an area config just to display a tooltip can be extremely cumbersome especially in larger maps.

Describe the solution you'd like
Add the following features:

1. Support the ability to have the value for a toolTip be a function in addition to the current text & jQuery options
2. Add a toolTip property to the standard mapster options in order to have a single option drive all toolTips

('img').mapster({
    ....
    toolTip: function(data) {
        // default applies to any area that is not explicitly specify its own toolTip
        return $(data.target).data('firstname');
    },
    areas: [
        {
            key: 'george',
            // george has a customized tooltip
            toolTip: $('<div/>').text('A tooltip for George')
        }        
    ]
});

Describe alternatives you've considered
N/A

Additional context
N/A