Created duration_to_vstr() - like duration_to_str(), but it appends a space
then a clarifying or modifying word (year[s], month[s], day[s], etc.). parse_byte_count() and parse_duration() now support a space between the numeric value (decimal digits) and the modifier. Hopefully this doesn't break anything, but it makes parse_duration() compatible with the strings generated with duration_to_vstr(). We might want a verbose version of byte_count_to_str() in the future, so I figured it made sense to go ahead and add the white-space skipping/parsing to prase_byte_count() now.