j2c/docs/plugins.md

A tiny CSS in JS solution.

## filter plugins

A filter plugin is declared as `{$filter: function setup(next, inline){ /* ... */ }}`

They are quite complex so we'll explain how they work by using examples of increasing complexity.

```JS
var takesRawNumber = [...]
{$fliter: function (next) {
    return {
        decl: function(prop, value) {
            if (
              !takesRawNumber.includes(prop)
              && (typeof(value) === 'number' || /^\d+$/.test(value.toString())
            ) {
                value = value + 'px'
            }
            next.decl(prop, value)
        }
    }
}}
```