only set environment to defaults if not already set (enables customiz…

[kda]

…ation without editing source)

This allows the particular PICKER_* environment variables to be set prior to the plugin being loaded to have priority over the defaults. This allows for overriding in personalized ${HOME}/.tmux.conf, without having to hack ${HOME}/.tmux/tmux-picker/tmux-picker.tmux directly as the documentation guides. This is useful, as it allows the customization in a shared .tmux.conf file, instead of having to hack each repository on each installation.