Add eventplot function.
Fix eventplot for 1d arguments.
Add alpha argument to eventplot().
Add test for eventplot().
Merge branch 'feature-eventplot' of github.com:jgosmann/prettyplotlib into jgosmann-feature-eventplot

* 'feature-eventplot' of github.com:jgosmann/prettyplotlib:
  Add test for eventplot().
  Add alpha argument to eventplot().
  Fix eventplot for 1d arguments.
  Add eventplot function.