Same as the other symbol download options, this one takes a list of all
nerdfont icons and wrangles them into shape to be usable with the emoji
picker. Can be used by invoking `bemoji -D nerd` or `bemoji -D all`.
Fixes #10.
Changed `-D` option to always download the lists supplied, regardless of
the database directory being empty or not.
This means you can download additional lists after the first program
run, or re-download lists later on. It *will* overwrite your custom
changes however if your files are called the same as the default list
names.
Additionally removed dependency on GNU version of cut and added some
simple tests for the download functionality.
Fixes #16.
