Introduce more options around working directory inheritance

[zx8]

Coming from iTerm, I have the following setup:

• For split panes, inherit the working directory from the originally-focused pane
• For new windows, use the home directory
• For new tabs, use the home directory

The only options that affect this behaviour at the moment are:

• working-directory – specify a static directory
• window-inherit-working-directory – opt in/out of inheritance for tabs/panes/windows

Perhaps the following new options could be introduced to control the behaviour at a more granular level:

• tab-inherit-working-directory
• pane-inherit-working-directory

[mitchellh]

Dupe: #1392 already accepted!