Add hotkey for magrittr's pipe

This inserts ` %>%`, a new line, then reindents on `CMD + SHIFT + .` or `META + SHIFT + .`
I think it's too aggressive to re-indent the whole buffer. You should either:

(a) just re-indent the magrittr expression; or 
(b) Figure out the indentation level by inspecting the current line and inserting the appropriate indentation. Note that to correctly insert indentation you need to refer to the current UI prefs (see here for an example: https://github.com/rstudio/rstudio/blob/master/src/gwt/src/org/rstudio/studio/client/workbench/views/source/editors/text/TextEditingTargetLatexFormatMenu.java#L130-L140)