Feature request: add support for '_' as thousands separator to math command

It would be nice if I could use '_' as a thousands separator as a parameter to the math command. I'd like to be able to write 2000 as 2_000. I'm not asking for the output to change. 

Python, JavaScript, Perl and Ruby all support '_' as a thousands separator. It makes it easier to read large numbers. 

Example of suggested usage:
```
$ math 2_000 + 2
2002
$
```

