`define()` bif

This adds the `define()` function, which allows you to create or overwrite a variable with a given name, passed as a string, onto current scope.

This bif can be useful on those cases in which you'd wish interpolation in variable names:

```stylus
prefix = 'border'
border = { color: #000, length: 1px, style: solid }

for prop, val in border
  define(prefix + '-' + prop, val)

body
  border: border-length border-style border-color
```

```css
body {
  border: 1px solid #000;
}
```