Syn with wordpress plugin svn repository.

Prefer download from:
https://wordpress.org/plugins/blank-footnotes/

Fixed plugin name

• Added note for Gutenberg
• Tested with Wordpress 5.0.1

• Added note for Gutenberg.
• Fixed style according to WordPress Coding Standards for
  PHP_CodeSniffer.
• Tested with Wordpress 5.0 classic editor

• Fix some strings

• Fix jetpack detection
• Fix translated string
• Fix button string
• Fix syntax for Wordpress coding standard

Fist version in public Wordpress plugins directory

• Added Documentation in PHPDoc format.
• Renamed some function with bfn_ preposition

This is a first public release.