Set MACOSX_DEPLOYMENT_TARGET so that older macs can run the mosh binaries

Closes #424
Add support for ECMA-48 escape sequence for italic

Closes #443
Implement bracketed paste mode

Allow bracketed paste mode-setting control sequences to be passed to the
outer terminal.

Signed-off-by: Barosl LEE <vcs@barosl.com>

Closes #430
mosh-server: Allow startup without PTY