Add scroll keymapping

[hrsh7th]

This PR aims to support scrolling floating windows.

[hrsh7th]

Should we add sample mapping for README.md?

[prabirshrestha]

We should add sample in doc.

rest looks good for merge.

[prabirshrestha]

What happens if floating window is not supported?

[hrsh7th]

This implementation does not handle preview windows. so if the floating window did not support, it just ignored.

[hrsh7th]

If we add setwinvar(winid, 'lsp_win', 1), we can handle only vim-lsp's windows.
Should I add lsp_win for output.vim and documentation.vim?

[prabirshrestha]

I’m ok adding that var.

[prabirshrestha]

@hrsh7th let me know if you had a chance to look at the comments.

[hrsh7th]

Sorry. I was losing a lot of time to fix my buggy completion engine... (it still buggy...)

For the time being, I'm back to finish this PR.

[prabirshrestha]

@hrsh7th is this good to merge now or still buggy?

[hrsh7th]

Ah. Sorry for my poor English.
This PR is not buggy. I think it ready to merge.

The buggy plugin was my auto-completion plugin (nvim-compe). I was taken a lot of time due to nvim-compe problems so I couldn't work on this PR.

[prabirshrestha]

Sounds good. Thanks for the PR.