Use relative font metrics and allow setting font size

[jkrumbiegel]

I haven't done this so far because docx doesn't have relative metrics (that's why there is indent_pt even though it's not great to hardcode that value). But after thinking some more about it, the solution is to use relative metrics everywhere and simply feed the absolute fontsize for docx in with to_docx(table; base_fontsize_pt = 12). Currently, tables in html look pretty bad when the base document changes fontsize from about 10 to 12 pt to something larger, like in a revealjs presentation.

This would also allow to choose larger fonts in some places, for a title, for example.