Refine behavior when cursor is in the middle of a path

[b0o]

In the case that the cursor isn’t at the end of a path:

/usr/local/foo/bar
          ^
          cursor

The current behavior is to use the full path as the zfzf target. Other options include:

• Only use the left-hand side (lhs), then re-append the right-hand side (rhs) after selection
• Ignore the rhs
• Use the lhs as the target, but only show subdirectories of the lhs that contain the path on the rhs (either as a direct or distant descendant). Similar to /usr/local/**/foo/bar.

Ideally, zfzf intelligently picks the option that makes the most sense based on context. And/or we could add new parameters to explicitly specify the desired behavior.