Implement flag to open a new tab

[brenobaptista]

I just asked mmstick on Reddit and apparently we don't have that, so he asked me to create this issue

I wanna open a new tab in cosmic-term programmatically because on my script I wanna open a new tab to authenticate instead of creating a second window.

gnome-terminal for example uses --tab, so maybe you could use that since current Pop!_OS users will come from GNOME