Move and merge vim plugin to fzf.vim repo

[balki]

For a new user, it is unnecessarily confusing to add two packages. As an end user I don't see why would someone want to just use fzf plugin without fzf.vim. There are no hard dependencies in fzf.vim, so even for other vim plugins depending on fzf should not be affected by the extra code added by fzf.vim. I see that there is also the option to set custom command prefix which can be used to avoid name collision.

Other advantages:

1. No need to have go code (which is effectively dead code but not obvious) in ~/.vim/pack/....
2. I see some vim issues in fzf repo while they belong to fzf.vim. Keeping them separate will be easier to manage.

Having to add two packages discourages new users to try this awesome plugin.