mosh-client: Make terminal escape character configurable

Uses environment variable MOSH_ESCAPE_KEY. Defaults to current Ctrl-^
which is somewhat problematic for many non-US keyboards.

Signed-off-by: Timo J. Rinne <tri@iki.fi>

Closes #425. Closes #215.
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>