Add CSS selectors to left- or rightalign slides

This MR adds two CSS selectors that allow users to left- or right-align slides:

## Left-aligned slide {.leftalign}

This sentence will be left-aligned

## Right-aligned slide {.rightalign}

This sentence will be right-aligned

requested review from @m300827

approved this merge request

mentioned in commit 7e870b7b

merged