Macos – iTerm/OSX Terminal: Change text margin from edge of window

Currently, I use iTerm on OS X. My terminal looks something along these lines:

I came across this picture of a terminal setup on GitHub, and I wanted to imitate it:

Specifically, I wanted to change the padding/margin along the left side/bottom side of the window. While the picture in question appears to utilize OSX's default Terminal, I'm hoping it can be done in iTerm as well, although an OSX Terminal-specific solution would work too.

I've tried looking through iTerm and OSX Terminal's preferences (especially under the "Window" section on iTerm, but I have looked elsewhere), and I have yet to find an option for adjusting where the characters start along the left side.

I've also tried to implement this programmatically in Bash:

bash -l | sed 's/^/ /'

This seems to work fairly well, although it only indents program output, not my prompt line (and I'm having trouble setting such a piped command as my login shell).

Also, it only adjusts stdout, not stderr and piping it with bash -l 2>&1 simply hides all of Bash's output altogether.

Have I just overlooked some setting in iTerm/OSX Terminal? Is there any other way to set an "inner text area" within the Terminal window (i.e. set padding along the sides)?