Add option to customize the List Of Acronyms format

[rchaput]

The List Of Acronyms (LoA) was previously rendered as a Definition List, which can be stylized in HTML/CSS, but not when using other output formats (PDF, DOCX, ...).

This PR:

• adds a new loa_format option to customize how the LoA is rendered;
• refactors the way we generate the LoA, to use this new option, resorting to Definition List as a default;
• adds a test to ensure the custom format works;
• updates the documentation accordingly.

Solves #19