Fix types, docs, examples in labeling

[kanitw]

Merging into the label feature branch.

I also created https://github.com/vega/vega-lite/milestone/73 as a milestone for label. Let's plan to fix all P1 before we release.

Add:

• bar_label, bar_horizontal_label
• (still buggy) stacked_bar_h/v_label, bar_label_negative, rule_extent_label
• rect_heatmap_label

Rename

• (old) line_color_label -> line_color_end_label
• label_multi_line -> line_color_label

[domoritz]

@kanitw @chanwutk what's the status of this pull request?

[chanwutk]

I actually have not worked on this for quite a while. I remember that our last discussion was on designing syntax and defaults for bar and line labeling. Seems like there are a lot of updates since then. I would be happy to meet, if that is helpful.