added option to exclude strings by ref
support excluding refs in read_strings_po
fixed default arg value; use passed val.
removed nargs which was messing things

With nargs, list looks like
[['a'], ['b']]
instead of
['a', 'b']
Report number of lines excluded instead of reporting each individual line
Merge branch 'optionalstrings'

This adds an option -x to exclude lines with specific reference.
Useful for excluding days or months from translation.