Stop applying plugin for newer versions of Vim

[casr]

I usually just let my plugin manager take care of updating and, unfortunately, I can miss some important updates such as this plugin is no longer necessary if Neovim and Vim are new enough. To be honest, the reason I looked at the repo was because I faced the issue described in #33 .

There can still be utility for this plugin for those users stuck on older versions of Vim.

With that in mind, I'd like to suggest detecting newer versions that natively support focus events and then bail early. Those behind on the times are therefore not negatively impacted and those that need what the plugin provides can happily continue to reap the benefits.

And, lastly, thanks for many years of service!

[lixiaoquan]

Latest vim and neovim 0.5 can auto reload changed file without this plugin, but I found with this plugin, the refreshing delay is much shorter. I am not sure whether it is caused by some configure but I feel this plugin is still useful.