Customizing visualization type for Vis

[dorisjlee]

Expose the ability to let users set their own visualization type.

1. Expose visualization type as a parameters in Vis e.g.) Vis(["Weight","Height"],df,mark="bar")
2. Expose the ability to set vis.mark publicly. Add errors and warning for invalid vis type for given intent. e.g.) vis.mark = "bar"
3. Ensure that Compiler logic complies with different types of mark. For example, the following bar chart seems ill-formatted for the given intent when mark is modified.