Respect $ZDOTDIR on installation

[Sneethe]

This conforms to ZSH behavior on where to put ZSH files.
As $ZDOTDIR is where things like .zshrc and .zsh_history is stored.

For majority of users $ZDOTDIR will = $HOME thus the default location is preserved.

If user has changed $ZDOTDIR to say $HOME/.config/zsh then upon this plugin's installation, .z will be in $HOME/.config/zsh

To avoid breakage for users who have set a custom $ZDOTDIR to something other than $HOME. but haven't changed the default database location of $HOME/.z
We do this:

: ${datafile:=$HOME/.z}
  [[ -f $datafile ]] || datafile=${ZDOTDIR:-$HOME}/.z

So even if $ZDOTDIR has been customized, we still default to the default database file location if it exists.

And thank you for making this plugin. Its very much appreciated.

[Sneethe]

And as for why. Well if a user has gone out of their way to configure $ZDOTDIR then its very likely they don't like their $HOME cluttered with dotfiles.

Also I felt it made sense for .z to be in the same path as .zsh_history

[Sneethe]

Just read through #20.
I think using 'XDG Base Directory Specification' just for a single file isn't appropriate. With #96 not even following the actual XDG spec.

setting datafile=$XDG_CONFIG_HOME/.z or $HOME/.local/state.z both of these aren't valid.

I do however agree with bavalpey that the number of people who use both zsh-z and bash's z simultaneously has to very small.

This PR would only break rupa/z if a user did the following steps in order:

• Starts using zsh-z
• Sets $ZDOTDIR to something
• Then decides to start using bash with rupa/z