Add custom title to bookmarks

# Idea: Add custom names to bookmarks

For example, part of my config is

```
let g:startify_bookmarks = [
\{'1': '~/.vimrc'},
\{'2': '~/.zshrc'},
\{'3': '~/.tmux.conf'},
\{'4': '~/.config/alacritty/alacritty.yml'},
\{'5': '~/.hammerspoon/init.lua'},
\{'6': '~/Documents/MD Notes/'}
\]
```

So at start, it shows up as

```
Bookmarks

[1] ~/.vimrc
[2] ~/.zshrc
[3] ~/.tmux.conf
[4] ~/.config/alacritty/alacritty.yml
[5] ~/.hammerspoon/init.lua
[6] ~/Documents/MD Notes/
```

# Example

We could have something like

```
let g:startify_bookmarks = [
\{'1': '~/.vimrc', 'Vimrc'},
\{'2': '~/.zshrc', 'Zshrc'},
\{'3': '~/.tmux.conf', 'Tmux config'},
\{'4': '~/.config/alacritty/alacritty.yml', 'Alacritty config'},
\{'5': '~/.hammerspoon/init.lua', 'Hammerspoon config'},
\{'6': '~/Documents/MD Notes/', 'Markdown notes'}
\]
```

So that it would show up as

```
Bookmarks

[1] Vimrc
[2] Zshrc
[3] Tmux config
[4] Alacritty config
[5] Hammerspoon config
[6] Markdown notes
```

The issue gives configuration examples and the expected bookmark display, but names no source files, tests, or entry points. Start by locating where startify_bookmarks is parsed and rendered, then verify that custom titles appear while existing bookmark paths still open correctly.

Written by the indexing model from the issue text.