pairwise comparisons - allow setting the p.value threshold (alpha)

[smaegol]

Hi, sometimes (in my case, especially when doing ggbetweenstats) there are so many significant pairwise comparisons that showing them makes a plot very crowdy. However, there are comparisons that could be easily excluded by setting the p.value threshold (alpha value) there than default 0.05. It would be especially useful for comparisons where groups are numerous, leading to low p.values even for small differences.

I can see that 0.05 is hard-coded in the ggsignif_adder. By changing it to the variable (like sig_threshold or whatever), passed from the main function, it would be easy to employ ggsignif filtering without manual invocation of ggsignif::geom_signif and pairwise_comparisons(), very useful also in the case of shiny apps.