Don't replace variables not specified in variables

[hughsk]

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:

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

Using:

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

Currently results in:

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:

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

Thanks! :)

[jonathanong]

can you add tests?

btw i recommend just using https://github.com/reworkcss/rework-vars since it actually somewhat complies with a spec

[jonathanong]

you still interested in this? i can just add you to the org if you want to maintain this