Ensure HTML entities are editable in nav labels.

Navigation label input now behaves consistently with post title input.

- Use `esc_textarea` instead of `esc_attr` when displaying in text input.  (We actually do want double escaping in that context).
- Consequently, don't use the post object passed to our metabox handler. The post title has already been formatted to edit (escaped).  Prevents storage of double escaped entities in database.
Nav label filtering in `bu_navigation_format_page`

Adds `bu_navigation_format_page_label` filter
Adds default "the_title" filters
Version bumps to 1.2.1
readme updates.
Merge branch 'release/1.2.1' into develop