Feature request: Add an option for linear-TSV-style writing/reading

[dvg-p4]

In the Linear TSV format, tabs, newlines, and backslashes in the data are represented by the strings \t, \n, and \\. (This has certain advantages, such as not having to scan every fields for quotes when parsing). Support for writing this format with vroom::vroom_format() would be particularly useful for us.