Add .markdown file extension to be processed as markdown

The node.js project uses .markdown and i wanted to use harp to preview the documentation.

I thought about just adding markdown to the processor list instead of added a whole new file and including the other file, but i liked that it was a automated process adding processors to the list.

```
helpers.processors["html"].forEach(function(sourceType){
  processors[sourceType] = require("./processors/" + sourceType)
})
```

like added markdown was another solution possible

```
processors["markdown"] = require("./processors/md");
```