Naming scheme for examples files non-friendly to Windows

[sogaiu]

As mentioned in this comment, some characters don't really work well as parts of filenames in typical Windows setups. One consequence of this might be that folks who are using Windows may be at a disadvantage working with this repository (including participants in this year's Janetuary examples activity).

This comment attempts to enumerate specifically which characters might be problematic.

So far /, *, and % receive special handling, but:

• <
• >
• :
• ?

don't seem to.

May be tweaking the code here to handle these additional characters is an appropriate modification?

If the code is changed after some files with the additional characters in their names get created, the relevant filenames probably should get changed appropriately as well.