Adding "imgClass" prop to the CldImage vue component

[alxistn]

Brief Summary of Changes

After submitting an issue to the repo (cf. https://github.com/cloudinary/cloudinary-vue/issues/145) , I am submitting this pull request to add the possibility to pass a css class down from the <cld-image> component to the generated <img> tag.

What does this PR address?

• [x] Github issue ( #145 )
• [ ] Refactoring
• [ ] New feature
• [ ] Bug fix
• [ ] Adds more tests

Are tests included?

• [ ] Yes
• [x] No

Reviewer, Please Note:

As of today there is no possible way to style the img tag using css class and the component <cld-image> because all css class added to the <cld-image> (eg. <cld-image class="my-css-class">) will apply to the <div> container not the <img> tag.