Feature request: Separate key bindings for enabled- and disabled search mode

- [x] I have read through the manual page (`man fzf`)
- [x] I have the latest version of fzf
- [x] I have searched through the existing issues

## Info

- OS
- [x] Linux
- [ ] Mac OS X
- [ ] Windows
- [ ] Etc.
- Shell
- [x] bash
- [ ] zsh
- [ ] fish

## Problem / Steps to reproduce

Thanks for `fzf`, it is amazing. It would be nice if one could also set up different keybindings for search mode and non-search mode, the latter of which I will call browse mode.

I would like to set up `fzf` to start in browse mode with search disabled and bind custom actions to keys (including letters without modifier keys and Enter). I would like to bind '/' to enable search mode and activate different key bindings (for example letters should just `put` themselves). In search mode Enter should also be changed to exit search mode and reactivate the keybindings of browse mode. The overall effect would be that it would behave a bit like `less` or `tig` does.

As far as I can tell currently this can not be achieved and I think it would be a nice addition. Please consider whether this is something that others could find useful too. Thanks!