Add --no-clear-on-start option

[phanen]

This can be used in shell integration, to avoid clear the cmdline
content

before:

after:

[junegunn]

Can you explain more about the screenshots? Which feature are you testing, and on which shell?

[phanen]

This is fish, I also tested in zsh.
Cursor is on |: nvim | filename1 filename2.
I trigger fzf shell integration to help me insert a new filename. Without this pr, filename1/filename2 is hidden. (clear by csi J)
This pr just avoid clear it when we enter.