add find_files method to Store

find_files will return an iterator that produces a series of Path::Tiny
objects. These paths can then be given to read_file() or read_document()
to produce the desired content.

This is a needed method for the Static app.
add open_file and write_file for filehandles

For efficient transfer of files between Stores, we need a filehandle we
can read line by line. open_file() gives us a filehandle that we can
then give to write_file().

This is needed for the Static app.