simple cli calculator

```
npm install calculator
cd calculator
npm start
```

or

```
npm install -g calculator
node-calc
```

When starting calculator, it starts a node repl with all of the following properties set as properties of the `GLOBAL`

object.
All Math properties are replaced with `Math.(prop)`

so you can write `var f = func('f(x) = sin(x)')`

instead of `var f = func('f(x) = Math.sin(x)')`

```
var f = func('f(x) = x*10 - 20')
f(3) //returns 10
f = func('f(x, t) = Math.pow(x, 2) + t + 1')
f(2, 4) // returns 9
```

params `([string|function], point to evaluate function)`

`derive('f(x) = x*x', 2) //evaluate derivative @ x = 2; returns 4`

params `([string|function], lower limit, upper limit)`

`derive('f(x) = x*x', 0, 1) //evaluate definite integral from x = 0 to x = 1; returns .333 = 1/3`

Set this property to determine how accurate the definite integral will be. Essentialy the "dx" in f(x)dx.
Default is `1/1000000`

Set this property to determine the distance between an estimated answer and a whole number that you wish cli calc to return the rounded number.
This is useful when you know the answer is a whole number and you don't want something like `1.99999999998384`

returned instead of `2`