Tooltips

[Sly1024]

Added configurable tooltips, because I needed them in my project: https://github.com/Sly1024/pigod.

Awesome charts, BTW!

[drewnoakes]

Very cool. Took a while to get to this, but I've fixed the conflicts and made a few tweaks such as:

• updating the tooltip during rendering, so values update during scrolling even if the mouse isn't moving
• deregistering mouse event handlers on stop()
• allowing the tooltip option to be modified while the chart is running (for the builder)
• updating the builder to allow playing with tooltips
• move positioning offset to CSS
• modifying format signature slightly, and specifying it fully in the .d.ts file
• lightening the default tooltip vertical line colour
• move binarySearch to Util

Thanks for the PR.

[drewnoakes]

Here's a screenshot from the builder: