add possibility to define options

It may looks too much, but last 6 commits is styling of readme/verb and docs, at all.
Actually you can provide almost everything through options - read fn, stat fn, filter fn, etc. Main purpose was to add option for filtering files. Maybe more tests needed for other available options, but this is good starting point.
Reason for that wish is because I want to use this module in `mustache-spec` where we have json and yaml files

And now with that change I can do something like this to export yaml specs
```js
var path = require('path');
var yamlSpecs = require('export-files')('./specs', false, {
  key: function(fp, opts) {
    return path.basename(fp);
  },
  yaml: function(fp, opts) {
    return require('read-yaml').sync(fp)
  },
  filter: function(fp, opts) {
    return /\.yml$/.test(path.basename(fp))
  }
});
```

Want to point also that verb's apidocs have some bugs (add some extra spaces), I'll go check it soon. Because of that "bug" I hardcode it to verb.md instead of `{%= apidocs("index.js") %}`.