Add a customization option: org-brain-narrow-to-entry

[vedang]

Commit Message:

If this option is set to t, org-brain-goto will also narrow the entry buffer to only show the entry, using org-narrow-to-subtree. This is useful to only focus on the entry, which is what the reader may want to do in most cases.

Explanation:

When I visit an entry, I don't want to see the rest of the contents of the buffer. This is especially true for deeply nested entries. This new custom var - org-narrow-to-subtree - let's me control the behavior of org-brain-goto to enable this. The custom var is turned off by default and does not affect the experience that users are familiar with.

[vedang]

Gentle Reminder for review / merge of this change :)