Add function declaration to a header file

[brobeson]

Write a vim plugin to extract a function declaration and add it to a header file.

Assumptions:

• The header and implementation files have the same name.
• The header and implementation files are in the same directory.

Design considerations:

• [ ] Include a variable to set the header file extension, default to .h
• [ ] Include an option to insert doxygen at the declaration.
• [ ] How to handle class methods, or functions in a namespace.
  • [ ] For class methods, make the declaration public, private, or protected.