Add a "admin-mode" parameter to the SpawnCommand structure

[mood-indigo]

I would like to suggest adding a boolean "admin-mode" parameter to the SpawnCommand structure; default is false. If the parameter is true, wezterm runs the command with elevated privileges; run as administrator in Windows and via "sudo" in Posix.

It would also be nice to have a new config.admin_mode = { ... } structure to specify the bg/fg colors, the cursor's colors and style, maybe a (possibly blinking) border around the window. The structure doesn't have to be complicated; just enough to remind the user that every commands she types in a admin-mode=true window will run at elevated privileges.