version bump 0.6.20: more flexible JSON

- sheet_to_json more flexible, sheet_to_row_object_array now just wraps it
o opts.header = 1 for array of arrays
o opts.header = 'A' for spreadsheet column labels
o custom opts.header array for custom labels
o opts.range = n starts from row n
o opts.range = range restricts writer to work within the specified range
- fixes #42