md5 file operation enhancements

[jmbarbone]

Connection objects should be allowed for inputs

• file_copy_md5() to accept a connection object
• write_file_md5() to accept a connection object
  • potentially some problem with line endings?
  • use open = "wb" in compress()
• mark_write_yaml() parameter improvements:
  • ordered_lists = TRUE, omap = ordered_lists
  • indent.mapping.sequence = TRUE for default
  • sep = "\n", line.sep = sep
  • figure out handlers (there's a default for boolean included)
  • indent = 2L
• mark_write_json() should default to a single flat line (pretty = FALSE)