Tooltip on overlay line charts and area charts

[light-and-salt]

Please merge #46 first. Branch-wise difference: zqu/rename...zqu/tooltip-on-layers

Tooltip is now working with overlay config for line and area. Fix #47

For line chart, when hovering on the points, we get all fields in tooltip:

When hovering on the line, we only get the string and boolean fields in tooltip:

Similarly, when hovering on the points on top of an area, we get all fields:

When hovering on the area itself, we only get the string and boolean fields:

[kanitw]

Please resolve conflict

[light-and-salt]

Rebased and the conflicts are gone!