Add last tab close options to back key (auto tab close) behavior

[sdothum]

i use the "close tab when it's already in the first page in history" behavior option.

When on the last tab (1), this will cause einkbro to exit.

i would like to be able to remain in einkbro when pressing Back at the head of the history chain.

Qutebrowser goes one step further and allows setting:

• ignore
• blank
• startpage
• default-page
• close

But i would be happy just to remain on the last page (ignore).