Use default `max_lines` when calling `up` & `down`

[HeyNonster]

Description

Both up and down only show 1 source line when called, I think it would be more useful if they provided more context. This removes the max_lines: 1 kwarg when they call show_src. max_lines in show_src will default to CONFIG[:show_src_lines]^1.

Big thanks to @st0012 for showing me and @bquorning where this behavior is defined.