v1.7.5

corner case: rule out cyclic filename change

This release adds codes to deal with the corner case, that a note has a
subtitle ending with "(1)", "(2)", "(3)" ....

So far this lead to filename endings that could have been confused with
the copy counter tag "(n)".

This release defines a new configuration variable
`copy_counter_extra_separator` which inserts an extra separator (`-`) in
case there could be confusion.