Enhance to work with custom css-tags

[digital-h]

I pretty much like your library. But unfortunately I ran into the problem, that I already have a style for my pagination with my dedicated css-classes. Therefor I needed a way to customize your output to match my css-classes.

I now came up with the following apprach, as I think it is most straightforward to handle it directly in the tag. The idea is: if a custom-css-tag is defined, then use it. Otherwise do the fallback to the default css-class.

I hope you like my approach and I would be glad if you would include it in your project, so that I can also work with future releases.

Hint: I only refactored the properties-file for german now. In case you want to merge this PR, these changes have to be adapted to the other languages as well.

Hope to hear from you soon!

[jpenren]

Another alternative can be overwrite the properties files in your own project. If do you place your custom properties files on src/main/resources/thymeleaf-spring-data-dialect/*.properties on project that uses the library, this will be used