[Podfile] Add a PodfileEditor that can add/update/remove REM sections
[Podfile] Add a PodfileEditor that can add/update/remove REM sections

`hasREMSection`: returns whether the file contains a REM section
`addREMSection`: adds a REM section to the end of the file
`updateREMSection`: rewrites all of the REM sections ex: maybe you moved the Podfile around and need to update the path in the REM section
`removeREMSection`: removes the REM section from the podfile

rnplay/rem,lwansbrough/rem