accept md and rmd files; refactor align, shrink, and calc_column_stats to work on address/visual range

[carywreams]

• plugin/rainbow_csv.vim converts RainbowAlign and RainbowShrink to use -range=%
• autoload/rainbow_csv.vim#csv_align
  • accepts either address or visual range
  • changes operational loops to have specific start/stop lines
  • emits error if an entire markdown or rmd file is submitted
• autoload/rainbow_csv.vim#csv_shrink
  • accepts either address or visual range
  • changes operational loops to have specific start/stop lines
  • emits error if an entire markdown or rmd file is submitted
• autoload/rainbow_csv.vim#calc_column_stats
  • accepts two new parameters, firstline and lastline
  • changes operational loops to have specific start/stop lines
• autoload/rainbow_csv.vim#s:named_syntax_map adds markdown and rmd filetype entries
• proposed README.md changes
• proposed doc/rainbow_csv.txt changes