utilize classnames library instead of manually joining classes

[capaj]

joining often creates unneeded spaces, which results in something like

class="tabs    is-fullwidth"

This could be much better if re-bulma used classnames. The same bug is #50

[bokuweb]

@capaj thanks. If you want to customize re-bulma, may I invite you to this repogitory as collaborator?

[capaj]

@bokuweb sure. I can't promise I'll have time to fix this soon, but I can at least start on this in a branch.

[gurre]

Please keep down the number of dependancies at all costs.

[capaj]

@gurre the lib is 50 lines of code and it will probably save around 100 LOC on this project. Well worth it.