Add Color Arguments to bullet_chart Function

[brichard1638]

Since the bullet_chart function is the principal Bullet Chart used in the bulletchartr package, it would be nice if the colors represented by the Low, Medium, High, Current, and Target elements could be user-defined. While the inclusion of this added functionality would increase the number of args in the function, it is believed that the value proposition would be well justified.

For example, the target element might be better served being green instead of red which in finance, typically is indicative of a negative value. One could make the argument that many stake holders subconsciously associate the color red with a negative outcome or data point.