fzf: Add DEFAULT_COMMAND and vim :Files mapping

Add FZF_DEFAULT_COMMAND that displays hidden files as well, but ignores
.git directories.
Add :Files mapping in vim. Because it is bound to <leader>f, the leader
key mapping had to be moved to the top of the file.

.config/vim/vimrc

@@ -1,3 +1,6 @@
+" Set leader key
+let mapleader = "\<Space>"
+
 " Plugins ######################################################################
 " Auto completion
 " needs vim >= 8.1.1719 to support features like popup and text property.
@@ -10,6 +13,7 @@ packadd! delimitMate
 " Fuzzy finder
 packadd! fzf
 packadd! fzf.vim
+nmap <leader>f :Files<CR>
 " Theme
 packadd! onedark.vim
 " Git information
@@ -83,8 +87,6 @@ set list
 set listchars=tab:>·
 
 " Keybindings ##################################################################
-" Set leader key
-let mapleader = "\<Space>"
 " Stop highlighting search result when pressing Return
 nnoremap <silent> <CR> :nohlsearch<CR><CR>