support for exponential notation

Right now typing `1e6` will perform `1 * e * 6`. I think it would be useful to first check whether the number should be evaluated using exponential notation.

```
> 1e6
1,000,000
```

```
> 1e12
1,000,000,000,000
```

Etc... Rust itself, Python and JavaScript support this and once you start using it, it's really hard to go without. 