add function to send current line to R and write its output into the line below

Hello Jakson,

to insert the output of an R command the vim-r-plugin has already the `RInsert` command. But often I find it cumbersome to use (because you have to write `:Rinsert print(foo)` into vims commandline).
This pull request introduces a new function `SendLineToRAndInsertOutput` that sends the current line to R, evaluates it and writes its output into the line below and comment it out:

    foo <- 1:10

becomes (after hitting *leader + lo*):

     foo <- 1:10
     #  [1]  1  2  3  4  5  6  7  8  9 10

Best wishes,

Sebastian