Design overview:

We are introducing multiple file path copy as a mode which can be toggled using
the keybind `^Y`. `^K` works as the individual entry selector. If the user wants
to select a range, (s)he can press `^Y` on the first entry and `^Y` on the last
entry.

We subscribe to notifications, so we need a fail-proof way to detect changes in
the directory contents. For example, if a file is deleted, it becomes difficult
to get the names of all the files in a range containing that file. If the file
is on a range boundary it would lead to wrong calculations. To handle this the
right way we use CRC8 checksum of all the visible entries in the directory. The
checksum is calculated based on the file information buffer. If the CRC changes
on a redraw(), we reset the multi-select mode.

New line (`\n`) works as the delimiter between file paths. Note that you may have
to disable IFS in the `NNN_COPIER` script to show file paths separated by spaces.

The replacement keys are:

^J - toggle du mode
^Y - quit

The change is done because ^S, ^Q keybinds are lost in the following case:
- start nnn
- navigate to a different directory
- spawn a shell
- exit the shell

The issue happens only with the 'special' keybinds like ^S, ^Q, ^Z... which get
their original shell interpretation back.

So we are replacing these 2 keybinds with 2 'non-special' combinations.

New keys:

- bookmark prompt (^B)
- pin directory (B)

The intention of the change is to facilitate opening the bookmark prompt in
navigate-as-you-type mode. The bookmark prompt is used more than the pin
current directory option.

This reverts commit 1e859eb137.
We will use atool on OS X as well, it can be brewed easily.

F2 is not very portable e.g., doesn't work over putty (default settings).

In certain workflows you know you would have revisit a directory. Mark it!

^Q would quit even from filter mode

1. Open directories in filter mode
2. Fix arrow keys not working after filter selection (due to invalid cur entry)
3. Support `Insert` key to clear filter prompt
4. Update documentation

1. Strip end spaces and slashes
2. Do NOT change dir if in the same dir
3. Don't go back beyond startup dir with '-'
4. Reset oldpath on cd (other than cd . and ..)
5. Reset filter
6. Update features in README

This change remaps the & key. However, / is more relevant for filter and & makes more sense wrt. address.