[Feature request] Support for env vars in monitor, bookmarks widgets

[mgrimace]

Hello, this request is to support inline env vars for both the monitor and bookmarks widgets.

The use-case is to avoid putting IPs or personal domains in plain-text on the glances.yml. This is useful for sharing configs/setups without having to redact information, and ideally safer practices. This is useful particularly if a user is using bookmarks or monitor to show homelab services.

For example,

                  - title: Overseerr
                    url: http://${SERVER_IP}:5055
                    icon: https://cdn.jsdelivr.net/gh/walkxcode/dashboard-icons/svg/overseerr.svg

or,

                 - title: Overseerr
                    url: https://overseer.${PERSONAL_DOMAIN}
                    icon: https://cdn.jsdelivr.net/gh/walkxcode/dashboard-icons/svg/overseerr.svg

This could potentially be useful for sensitive embedded keys, tokens, etc. as well, e.g.,url: https://connect.raspberrypi.com/devices/${RPI_2_UUID}/remote-shell-session

[svilenmarkov]

Hey, I'll get this added in v0.7.0.