Don't replace variables not specified in `variables`

Unsure if it's the current intended behavior, but if a variable is falsey or undefined, it'll just remove the `$` symbol and leave the variable name there.

For example:

``` css
body {
  margin: $body-margin;
  padding: $body-padding;
}
```

Using:

``` javascript
rework(str).use(variables({
  'body-margin': '20px'
})).toString()
```

Currently results in:

``` css
body {
  margin: 20px;
  padding: body-padding;
}
```

Meaning that any future applications of `rework-variables` anywhere else won't be able to pick up the missing variable and replace it. This patch changes the ouptut of the above example to be:

``` css
body {
  margin: 20px;
  padding: $body-padding;
}
```