request: option to force use of 64 bit hash function

[ctb]

For testing and interoperability reasons, I'd like to be able to force mash to always use the 64 bit hash function. Would it be possible to add a command line option for this?

(I'm also separately having trouble compiling mash on my own mac laptop, otherwise this might have been a pull request :)

[ctb]

ref #27 for interoperability stuff