Moved magrittr to Imports, and added the pipe in importFrom

[ColinFay]

These changes allow using the package without having to explicitly load the magrittr package. As this package doesn't work without having to library(magrittr) before, it makes sense to have magrittr listed in the dependecies.

With the current version, if I open a fresh R session, the package doesn't work:

library(backpipe)
mean %<% 1:5
#> Error in 1:5 %>% mean: could not find function "%>%"

^{Created on 2019-01-09 by the reprex package (v0.2.1)}

This problem is addressed in this PR :

(with the new version) :

library(backpipe)
mean %<% 1:5
#> [1] 3

^{Created on 2019-01-09 by the reprex package (v0.2.1)}