ppseq 0.2.0

• added weighting options to optimize_design() (Closed Issue #7)
• added informative messaging when no results returned from optimize_design() (Closed Issue #14)
• upgraded package website to bootstrap 5 and changed some styling
• added two-sample randomized trial vignette
• added calc_next() function to calculate probability of response in next patient (Closed Issue #2)
• fixed a bug in the default arguments to optimize_design() (Closed Issue #12)
• added efficacy monitoring option, default is still futility monitoring (Closed Issue #3)