Add styling parameter for multi-column glossaries

[quachpas]

Before this change, group-heading-level defaulted to reusing the last heading level found in the document. This is unlikely to always work for users. This new parameter allows users to override the default.

Additionally, to give users some ability to style the output without needing to override user-print-glossary, the new num-columns parameter gives users the ability to control the number of columns groups are printed in.

See #60