renamed: stdin/out converters start with filter.
filter-to-7bit is now filter-chr for flexibility.
Implemented filters for different character sets.

Right now this still yields the same results like previous versions of this
script. However, I already implemented routines to restrict input to 7bit
(us-ascii) or only printable ascii characters or only word characters. Each
preset is more restrictive than its predecessor.
Also, the replacement character will be configurable in a future version
including character deletion instead of replacement.