Set ls alias with color without a ~/.dircolors file

This commit adds a condition to the 'if' in alias.zsh to check for the
'dircolors' command without requiring a ~/.dircolors file. This will use
the default system colors without requiring the user to add a file to
their $HOME directory.
Change default LESS options

Customize the LESS default options to be more to my liking.
Add personal theme
Add zstyle option to set default opts for ls alias

This commit adds a zstyle to ':omz:alias:ls' in opts for the user to
define any defaults command-line arguments to pass to 'ls' in an alias.
Improve checking for OS in 'ls' alias code

Improve the if statement when setting 'ls' alias for color. On Linux,
'dircolors' is used to set the colors, optionally using the user's
$HOME/.dir_colors. Otherwise the OSX/BSD style is used.
Don't hard code options other than --color for 'ls'

The user can set default options for 'ls' using the ':omz:alias:ls' opts
zstyle, so remove unnecessary hard-coded options.
Don't share history

Remove the SHARE_HISTORY option from history.zsh. History from other
sessions can still be loaded manually with 'fc -R'.
Change completion cache to use XDG_CACHE_HOME

Change the dump and cache file locations to be stored under
XDG_CACHE_HOME/zsh.
Set compdump file location explicitly

Set the compdump file location explicitly; it will be created under the
XDG_CACHE_HOME directory.
Fix incorrect compdump directory

Fix compdump file location to be under ~/.cache/zsh/
Change name of compdump file to remove z prefix

Since the compdump file is now stored under .cache/zsh/, there is no
reason to prefix the file name with 'z', so just call it 'compdump'.
Ensure ~/.cache/zsh directory is created
Put zsh history under ~/.cache/zsh
Fix buggy terminal precmd

Make sure that the precmd only produces output if the ':omz:terminal'
auto-tile style is NOT set.

Actually I'm not really sure why it only breaks on Xubuntu 10.04 but not
on 11.04 -- a buggy xfce4-terminal?
Don't use extended globbing

Disable EXTENDED_GLOB; I don't use the functionality and it doesn't play
well with git.
Only add user-specific directories to PATH etc

Only set user-specific directories in *PATH; inherit global values from
the system zsh configuration. Also remove a bunch of paths that don't
make sense for my system.
Add gen-pass plugin

The gen-pass plugin implements a stateless password management system.
It provides a `gen-pass` function which automatically generates a
site-specific password using information in a password file and a master
password/phrase.
Merge changes from 'upstream' (sorin's branch)

Merge changes since 3/1 from sorin-ionescu's branch with my changes.
I've changed 'master' in my fork to point to sorin's master, and will be
keeping it in sync with upstream; my changes will be made to the
'cordarei' branch, with changes from upstream merged in periodically.
Tweak default `less` options

Don't set `-w` or `-g` options
Merge changes from 'upstream' (sorin's fork)

Merge changes from sorin-ionescu's OMZ, especially the recent
modularization. All plugins as well as former core functionality are now
separated into modules.

After this merge, the call to `compinit` (formerly in /init.zsh, now in
modules/completion/init.zsh) no longer sets the dump file (with `-d
$dumpfile`); I need to look into adding it back or using the default.