Add ALT-ENTER to paste selected items into the buffer

- New fzf#vim#paste inserts selections instead of opening them
  - First item after cursor (space-padded), rest on following lines
  - Leading whitespace stripped only when pasting after text
- Commands opt in via `_paste`; key configurable with g:fzf_vim.paste_key
- Skip the key when buffer is not &modifiable
- Per command, paste:
  - path: Files, GFiles, GFiles?, Buffers, History, Locate
  - line text: Lines, BLines, Marks, Changes
  - matched text: Rg, Ag, Grep
  - tag name: Tags, BTags
  - commit hash: Commits, BCommits

README.md

@@ -96,6 +96,12 @@ Commands
 
 - Most commands support `CTRL-T` / `CTRL-X` / `CTRL-V` key
   bindings to open in a new tab, a new split, or in a new vertical split
+- Most commands support `ALT-ENTER` to insert the selected items into the
+  current buffer instead of opening them. The inserted text depends on the
+  command: file path (`:Files`, `:GFiles`, `:Buffers`, `:History`, `:Locate`),
+  line content (`:Lines`, `:BLines`, `:Marks`, `:Changes`), matched text
+  (`:Rg`, `:Ag`, `:Grep`), tag name (`:Tags`, `:BTags`), or commit hash
+  (`:Commits`, `:BCommits`)
 - Bang-versions of the commands (e.g. `Ag!`) will open fzf in fullscreen
 - You can set `g:fzf_vim.command_prefix` to give the same prefix to the commands
     - e.g. `let g:fzf_vim.command_prefix = 'Fzf'` and you have `FzfFiles`, etc.
@@ -156,6 +162,17 @@ let g:fzf_vim.preview_window = []
 "   let g:fzf_vim.preview_bash = 'C:\Git\bin\bash.exe'
 ```
 
+#### Paste key
+
+Most commands support a key to insert the selected items into the current
+buffer instead of opening them (see the command list above for what each
+command inserts). Customize it with `g:fzf_vim.paste_key`.
+
+```vim
+" Key to insert the selected items into the current buffer (default: 'alt-enter')
+let g:fzf_vim.paste_key = 'alt-enter'
+```
+
 #### Command-level options
 
 ```vim