added option for unweighted circles

because some sets lead to awkward scaled venn diagrams, the option
to draw unweighted circles was needed. This is specified at the
command line
fixed make_a_list function bug for empty lines in files

when a filename is passed to make_a_list, if the file contained
an empty line it would return a null value that frequently caused
errors (or caused difficulties in interpretation with common_path)
To fix this, all empty lines in a file are now skipped.