Choose sanitizing behavior of max_len

[thombashi]

For now, sanitizing functions will trim the trailing string when the byte length exceeds the max_len.
This behavior may not be preferable in some cases.
Make it possible to choose the behavior of sanitizing when a byte length of the value exceeds max_len and change the default behavior.

Choices of behavior:

• KEEP: do nothing
• TRIM: trim the trailing string (current default)
• TRIM_NAME: trim the trailing string except for the file extension
• ERROR: raise an exception (new default)

[exislow]

This function would be awesome!