iterm2: Update integration, add iterm2_clear_captured_output() function
iterm2: Update integration, add iterm2_clear_captured_output() function

This brings our integration in line with upstream v12 (don't cache the
host name; we simply use $HOST now), fixes some typos, enhances the test
for the iterm2_print_user_vars() function and introduces a ax-zsh
specific new iterm2_clear_captured_output() function.

alexbarton/ax-zsh