Refine integer format and add scientific notation to floats.

Some examples:

```toml
# fractional
+1.0
3.1415
-0.01

# exponent
5e+22
1e6
-2E-2

# both
6.626e-34
```
This doesn't explicitly say that "the fractional part must come before the exponent part". I guess that's sort of obvious, but having things stated explicitly might be good...