Allow negative left/right margins

[bertfrees]

Supported in Liblouis based renderer but not yet in Dotify based renderer.

Depends on:

• [ ] https://github.com/joeha480/obfl/issues/30

[bertfrees]

For now, negative values for margin-left and margin-right are not required in Dotify, I have solved the problem in XSLT.

This pull request also fixes https://github.com/snaekobbi/pipeline-mod-braille/issues/32.

[bertfrees]

Rebased