Add option to customize popup window corners

The default corners are round and may not look so good with some
font, add this option g.lazygit_floating_window_corner_chars to
allow user to customize the corner characters.

README.md

@@ -32,6 +32,7 @@ The following are configuration options and their defaults.
 ```vim
 let g:lazygit_floating_window_winblend = 0 " transparency of floating window
 let g:lazygit_floating_window_scaling_factor = 0.9 " scaling factor for floating window
+let g:lazygit_use_neovim_remote = ['╭', '╮', '╰', '╯'] " customize lazygit popup window corner characters
 ```
 
 Call `:LazyGit` to start a floating window with `lazygit`.