Add support for magrittr

[g4challenge]

Please add reformatting for magrittr pipe operator %>%.

For example:
result <- mydata %>% filter(criterion) %>% group_by(mygroup) %>% summarise(summand = sum(count))

should be converted to something like:

mydata %>%
  filter(criterion) %>%
    group_by(mygroup) %>%
      summarise(summand = sum(count)) -> results

[edlee123]

Another example:

testfunc = function(){

  testout <- mtcars %>%
    mutate(mpgXcyl = mpg * cyl) %>%
    arrange(desc(disp), hp) %$% 
    carb
  return(testout)
}

> tidy_source()
Error in parse(text = lines, keep.source = TRUE) : 
  <text>:8:0: unexpected end of input
6:     carb
7:   return(testout)
  ^
> 

[yihui]

My reply would be the same as in #53...