cat offset and large files

[richardschneider]

The cat command/API now accepts an offset and length.

Files bigger than 2^32 bytes can not be read because the offset is only an int not a long

offset and possibly length should accept long values.

[Stebalien]

This will, unfortunately, require changes to the commands lib. However, you're absolutely right.

[overbool]

Hey @Stebalien, I think modifying IntOption/UintOption to Int64Option/Uint64Option maybe can solve this issue?

[Stebalien]

We'd probably want to add a type instead of replacing the existing one.