Allow Selection of Bars by Area

This PR generalizes `selectBar` to allow you to select all bars that fall under an area (the current functionality only allows you to select the bar under a single point). This is useful for drag interactions. Thanks -
The above docstring seems to be outdated; it lists 5 arguments, but this function only takes 3.

Also, you may want to use the proper type signature of this function, which is
```
public selectBar(xValOrExtent: IExtent, yValOrExtent: IExtent, select: boolean): D3.Selection;
public selectBar(xValOrExtent: number, yValOrExtent: IExtent, select: boolean): D3.Selection;
public selectBar(xValOrExtent: IExtent, yValOrExtent: number, select: boolean): D3.Selection;
public selectBar(xValOrExtent: number, yValOrExtent: number, select: boolean): D3.Selection;
public selectBar(xValOrExtent: any, yValOrExtent: any, select = true): D3.Selection {
  ...
}
```
but then again, that's pretty wordy.