Added lasso

The lasso can be used to draw loops around elements to tag them. The tags that are added are data properties:
- possible: a boolean value. While using the lasso, it is true for an element if the element would be selected if the lasso was completed in it's current state. It is false otherwise.
- selected: a boolean value. When the lasso is completed, elements that are inside the loop are set to true. The property is false otherwise.

The lasso is useful for situations like brushing scatterplots with complex shapes. An example is demonstrated at http://bl.ocks.org/skokenes/511c5b658c405ad68941 .

The style of the lasso can be controlled via CSS. The README file in the lasso folder contains more information on the parameters and use of the lasso.