Proposition : enhancement of tag sub/sup

Proposition helps to avoid bad line-height rendering caused by sub and sup tags. (use these tags on big blocks of texts with and without the fix to see the difference)

In french : la proposition aide à éviter des interlignages disgracieux causés par les balises sup et sub. ;)
Merge pull request #15 from nico3333fr/patch-1

Proposition : enhancement of tag sub/sup