eulerr 1.0.0

New features

• Final optimization routines have been completely rewritten in C++ using Rcpp
  and RcppArmadillo.
• Switched to the cost function from EulerAPE for the default optimization
  target but added the posibility to choose cost function via a cost argument
  (currently eulerAPE or venneuler).
• Added the option to produce conditional eulerr plots via a by argument to
  eulerr. The result is a list of euler diagrams that can be plotted
  in a grid arrangement via a new plot method.
• Improved label placement by using a two-dimensional kernel density estimation
  instead of means to calculate label centers.

Bug fixes and minor improvements

• Cleaned up typos and grammar errors in the Introduction to eulerr vignette.
• Added mar argument to plot.eulerr with a default that produces
  symmetric margins.
• Corrected the implementation of the stress statistic from venneuler.
• Switched to Vogel sampling to generate points to choose label positions from.
• Minor clean up and performance fixes all around.
• Added a print.eulerr method.
• Updated vignette to cover new features and changes.