Fix sstable.baseline

[trinhdhk]

Fix sstable.baseline (not summarising z)

[trinhdhk]

Fix:

• sstable.baseline not summarising z when pooledGroup == FALSE.
• disable implicit arms sorting when arm.var is of class factor. It is not reasonable to do so. Behaviours with character-type arm.var are left in-act.

Add:

• sstable.ae now has abilities to group variables into groups and sorting aes based on number of cases and p value.
• New functions: ssformat, ss_huxtable, ss_flextable to draw tables when flextable == FALSE in the original function. Relevant dependencies are needed.
• New helpers: ht_theme_markdown for huxtable markdown-friendly theme and ht_theme_kable for kable-cloned one.

All backward compatibility is supposed to be maintained.