FR: write_over() improvements

[krlmlr]

1. Preserve line ending. Otherwise we are too much dependent on the "eol" Git config settings. I have an implementation for transforming a file preserving line endings in {enc}, but I'm retiring it: https://github.com/krlmlr/enc/blob/master/R/io.R#L110.

2. Consider adding an ask argument that defaults to interactive(), so that other tools can call this function to unconditionally overwrite.

[hadley]

We also need to handle the case where the file doesn't yet exist; possible by reading the line endings from the DESCRIPTION (not sure where to look if the project isn't a package).