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.

plugin/lazygit.vim

@@ -19,6 +19,10 @@ if !exists('g:lazygit_use_neovim_remote')
   let g:lazygit_use_neovim_remote = 1
 endif
 
+if !exists('g:lazygit_floating_window_corner_chars')
+  let g:lazygit_floating_window_corner_chars = ['╭', '╮', '╰', '╯']
+endif
+
 command! LazyGit lua require'lazygit'.lazygit()
 
 command! LazyGitFilter lua require'lazygit'.lazygitfilter()