Add option to always apply score id suffix

[igneus]

When gly gabc or gly preview process a gly file file.gly, the resulting gabc files are named

• file.gabc if the gly file contains only one score
• file_0.gabc etc. otherwise (for scores defining header field id it's value is used instead of the number)

Sometimes it would be useful to have an option forcing gly to always apply the number/score ID, regardless of count of scores contained in the gly file.