Functions over history results

Functions like (sum, avg, etc.) over entire history of results.

`sum(*)` (or any other special character) should produce the sum of all results in the current session.

Or the way `bc` does it. Mark result as a variable then use the variable `tip` later in a sum with other variables.

```
bill=10+20+30+40+50
tip=bill*10/100
bill+tip
```