text: Title: Upgrade manual_legend to match line/marker style of plotted points
  Body: **Describe the solution you'd like**
  Upgrade `manual_legend` to match line/marker style of plotted points by enabling simple circles and lines with Line2D patches.
  
  **Is your feature request related to a problem? Please describe.**
  Right now `manual_legend` draws the patches as rectangles and cannot take into account the line or scatter plot properties (e.g. line style or marker style). It would be nice to add [Line2D patches](https://matplotlib.org/gallery/text_labels_and_annotations/custom_legends.html) that would enable the legend to show simple circles or lines to match the marker/line style the user has selected for the plotted points.
  
  ![image](https://user-images.githubusercontent.com/8760385/44339285-c851ab80-a44e-11e8-9c31-2a28944fa709.png)
  
  _Note: This issue is a follow-on to #564, where @bbengfort added a `manual_legend` to the newly created `yellowbrick.draw` module, that enabled us to update the visualizers that implement some kind of scatter plot and have an `alpha` param that affects the opacity/translucency of the plotted points. Now with the `manual_legend`, the colors in the legend no longer become translucent when `alpha` is decreased._