Update webassets plugin to allow user to pass configuration settings to webassets

Found that the webassets plugin didn't allow the user to modify any of the configuration settings. This is important for instance, on Windows in order to call NodeJS tools you have to call it like: `lessc.cmd` or `stylus.cmd` or it won't run.  

But it's more than that, because if you are using other tools you can alter the settings as well. This allows for a `ASSET_CONFIG` setting that you can add these items.

Plugin Documentation is updated.

Also removed language about DEBUG not compiling CSS for Less and Sass since as of at least webassets 0.8 this is no longer an issue.