Add mapping parameter to draw function

Allow mapping variables to ```colour```, ```fill```, ```alpha```, etc. by adding a ```mapping``` parameter to the ```draw``` function, while keeping the default of ```colour = "grey30"``` in case colour is neither mapped to a variable nor given as a constant argument. The default of ```alpha = 1``` is standard anyway and therefore not needed. 

Example: 
```
prodplot(happy, ~ happy + marital + sex, mapping=aes(fill=sex, alpha=happy))
```