Option to specify the encoding used for filename conversions

[boredzo]

impluse currently assumes that all volumes are MacRoman. It's got some infrastructure internally for dealing with other encodings but it isn't currently being exercised.

#16 covers accessing saved filename encoding data when it's present, but it isn't necessarily (particularly since the field was removed in Mac OS 8). We need a way for users to be able to specify a default encoding.

The order in which impluse tries to decode names should be:

1. Encoding saved in Finder info (Read the fdScript field where it exists #16)
2. Default encoding specified by user
3. MacRoman