[FEATURE] support for normal CDF

I think it would be cool if this had a desmos-like way of defining distributions and using them.

For example, object oriented-ish:
```
D = normaldist(1, 2)
D.cdf(-10000, 5)
```
Or purely functional:
```
normcdf(1, 2, -1000, 5)
```

Would be neat+helpful. Could open the door for a lot more features.