Add fzf-z widget

[deeuu]

Hi,

Thanks for maintaining this xontrib, great to see the subprocess workaround for using fzf with xonsh!

This PR adds a new widget that searches the combination of (displayed in this order):

• The file/dir listing under the working directory
• Any paths listed in a 'bookmark' file located at $SHELL_BOOKMARKS (defaults to ~/.shell_bookmarks)
• The output of z -l ..

It is loosely based on the fzf-z plugin for zsh.

It might be too subjective, based on my needs, so let me know if you think the widget would be better served as a dedicated xontrib.

Will add to the readme if you think it would make a nice addition!

[deeuu]

Hey @laloch, have made a few updates, see what you think.

Notable changes:

• I decided to drop the cwd listing because we already have the fzf_file and fzf_dir - no need to be greedy!
• Use the cursor prefix as fzf query argument.

[deeuu]

@laloch deciding to close for now as I want to consider a few additional features, e.g. auto-cd if the command buffer is empty when calling the widget.

[laloch]

You could have only turn this into a draft.