v1.2.0 - simplify StdioSerial; revert Print::println() line terminator, and make it configurable

• 1.2.0 (2021-12-29)
  • Simplify StdioSerial class, see
    Issue#43.
    • Replace input ring buffer with a buffer of one character.
    • Wire StdioSerial::write(uint8_t) directly to Posix write(),
      by-passing the <stdio.h> buffer. flush() is no longer necessary.
    • Thanks to @felias-fogg.
  • Revert Breaking Change Made in v1.1.0 Revert 432e304, so that
    Print::writeln() writes \r\n again by default.
    • Fixes Issue#45.
    • Add Print::setLineModeNormal() and Print::setLineModeUnix()
      methods to control the line termination behavior.
    • See README.md#UnixLineMode for usage info.