flatpak install: support installing from multiple urls

[soredake]

Linux distribution and version

Kubuntu 20.04

Flatpak version

Flatpak 1.8.2 from ppa

Description of the problem

flatpak does not support installing from multiple urls

❯ flatpak install -y https://flathub.org/beta-repo/appstream/com.google.Chrome.flatpakref https://flatpak.citra-emu.org/citra-canary.flatpakref
error: Too many arguments

See 'flatpak install --help'

Steps to reproduce

Run command flatpak install -y https://flathub.org/beta-repo/appstream/com.google.Chrome.flatpakref https://flatpak.citra-emu.org/citra-canary.flatpakref