append_save M-file function

 Objective: be able to add variables to existing save files. Works for
 all the types of save files that "save" supports.
 
 Input: 
 1) A string which specifies the existing save file.
 2) The options you need to pass to the 'save' function to save to the
 file type that you want.
 3) A 1x2 cell, with the first element being a string representation
 of the variable/symbol that you're trying to add, followed by the
 actual variable/symbol itself.
 4) Any number of additional 1x2 cells, following the same format as
 the 3rd argument specified immediately before this one.

 Output:
 Currently, none. But there might be some debugging / error-code
 messages in the future.

 Example:
 octave> B = ones(2,2);
 octave> append_save( "test.txt", "-binary", {"B", B } )

Package: io