Support "invalid" path characters in keys

[HakanL]

I had a key that used : and it caused a path error when BinaryRage was trying to create the subfolders with that characters. I suggest that the key is filtered before creating folders.

[tormaroe]

We can use Path.GetInvalidFileNameChars() and Path.GetInvalidPathChars() to identify and remove invalid chars before persisting in an OS-safe way.

My only concern is that this could result in unwanted key collisions. Maybe it could be a configurable option?

Another solution would be to throw an appropriate exception when key contains an invalid character, and then also provide a method the user can use to sanitize the key (filter out the invalids).

[HakanL]

It would be safer to escape those characters.