add tests, add `export-files`

Tests are dynamically with `forOwn` for every file in `lib/` you can [see here](https://github.com/tunnckoCore/markdown-utils/blob/master/test/test.js#L27-38) what I mean.

Fixtures are just array with arguments to be passed to utilNameFunction. e.g.
**test/fixtures/image.js**
```js
module.exports = [
  'Build Status',
  'https://travis-ci.org/jonschlinkert/template.svg',
  'This is title text of image!'
];
```
**.image actualizing**
```js
var actual = utilName.apply(utilName, fixture)
```
**test/expected/image.md**
```md
![Build Status](https://travis-ci.org/jonschlinkert/template.svg "This is title text of image!")
```

Is it too much? I think looks very pretty and nice (inlove), LOL!