Need write_file and write_lines

[hadley]

To do reverse of read_file() and read_lines() (should always write as utf-8)

[jennybc]

Could these return the path to the file they just wrote?

But I also see that would be inconsistent with write_delim() et al, which return the data frame they just wrote.

[hadley]

@jennybc I'm pretty confident now that the general principle is that if a function is called primarily for its side-effects, then it should invisibly return it's first argument.