Extending Mineral-Overlay to support multiple blocks.

With this commit the Mineral overlay is extended to multiple blocks.

A usecase to demonstrate a possible application of the extended functionality:

**"Rails Overlay that draws only the rails that are on Cobblestone for a subway map."**

With this patch it is very easy to achive that:

```python
MineralOverlay(minerals=[(((0, 0, 0, 66), (0, -1, 0, 4)), (255, 0, 0, 255)),
                         (((0, 0, 0, 27), (0, -1, 0, 4)), (0, 255, 0, 255))])
```

In this case the overlay will be red for rails on cobblestone and green for powerrails on cobblestone.
The syntax is `(<tuple of conditions>, <target color>)`
 * where `<target color>` is a 4 tuple with a `(r, g, b, a)` color
 * and `<tuple of conditions>` is a tuple with an arbitrary number of conditions with the following syntax:
`((relx, rely, relz, blkid), ...)` where the `rel<>` parameters specify the relative coordinates to the block that is checked if it matches bklid.

In the example the fist tuple `(0,0,0,66)` checks if at the current position is a
rail while `(0,-1,0,4)` checks if at one below the current position is a cobblestone.
If both are true then the color `(255, 0, 0, 255)` is used.

To not break compatibility the old notation with `(mineral, (r, g, b))` is still valid.

fixes overviewer/Minecraft-Overviewer#556 and fixes overviewer/Minecraft-Overviewer#787