pmplots 0.3.4

• pm_grid() now uses 'patchwork' to arrange plots; 'patchwork' becomes
  a suggested package and 'cowplot' is dropped #49.

• scatt() gains alpha argument with default provided by pm_opts$scatter.alpha
  #42, #48.

• split_plot() gains labeller argument which gets passed to facet_wrap()
  #43, #47.

• label_tex() replaces label_parse_label() for labeling facets via
  latex2exp::TeX(); label_parse_label() remains as an alias #47.

• label_fun argument for wrap plots is renamed labeller; applies to
  the following plots: wrap_cont_cont(), wrap_hist(), wrap_cont_cat(),
  and any plot built on these (see ?wrap_plots) #43, #47.

• Fix bug where points overlaid on boxplots were jittered in both directions;
  jitter is now only in x-direction #45, #46.