New option to sleep for each pane creation

[greymd]

There have been several Issues (#167, #173) reporting: problems when creating named pipes simultaneously.
The root cause is not known, but it is suggested that the problem is caused by an environment in which multiple named pipes are created at the same time.

It might be nice to have new options to facilitate problem isolation and provide temporary workarounds.

So I thought it would be nice to have an option to sleep the process for a certain amount of time after each tmux pane creation.
The Long option is a good choice since it is not likely to be used frequently.

Example:

$ xpanes --spawn-interval 0.1 -s {1..5}

Internally, the value of the option is just given to sleep command on that environment.
sleep 0.1

Please note that whether sleep supports floating number or not is depending on the environment ref.
GNU sleep supports it but other may not.

[greymd]

This idea can be the workaround for #157 as well.

[greymd]

Released in v4.1.4. Closed