Per-Directory-History

This is an implementation of per directory history for ZSH:

Typed commands are saved both in a global history, as well as a per-directory history.
A hotkey (AltL by default) toggles the current history, used for ZSH history navigation and search.
Directory histories are saved in a hierarchy under your home directory mirroring the filesystem hierarchy.

This implementation is loosely based on the implementation by Jim Hester. Notable differences:

Works smoothly with very large (global) history files. The global history file is only read in full when switching to it with the hotkey.
Works smoothly with share_history (commands are flushed to history files immediately, and become visible in other shell instances).
Some subjectively saner defaults and namespacing fixes.

Usage

Load this script into your interactive ZSH session:
```
% source zsh-per-directory-history.zsh
```
The default mode is per-directory history, interact with your history as normal.
Press AltL (lowercase) to toggle between local and global histories.

PER_DIRECTORY_HISTORY_TOGGLE is a shell variable defining the key combination which toggles between directory and global histories.
PER_DIRECTORY_HISTORY_BASE is a shell variable which defines the base directory in which the directory histories are stored.
PER_DIRECTORY_HISTORY_FILE is a shell variable which defines the name of the files which will contain the directory histories. It should be set to something that's unlikely to occur as a path component in any paths you intend to visit on your filesystem.
PER_DIRECTORY_HISTORY_NEW_PROMPT is a shell variable which defines if new prompt is created when the history is toggled. Set to false to only show a message below the prompt.
per-directory-history-toggle-history is the function to toggle the history mode.