Adding support for build.json "replace" function for CSS-Files

I needed replace function for CSS-files, defined in build.json. With this change it should be possible to use the syntax for CSS-files as you do for JS-files.

sample code: 
```json
...
, "p-gui-css-styles": {
      "cssfiles": [
        "style.css"
      ] 
      , "replace": {
        "@amber-off-16@": "background-position: -310px -46px;"
        , "@amber-on-16@": "background-position: -161px -17px;"
      }
      , "config" : {
        "type": "css"
      }
    }	
...
```