Support Mix release commands

[jjcarstens]

Resolves #91

Allows one to specify a mix release command to use with --mix-{CMD}

This is currently producing a Seg fault, but at least getting it up for someone better at C than me to look at why. (Or I will revisit when time allows)

[jjcarstens]

This is ready for a good review:

• implemented default command in the trailer. It is configurable from the release and defaults to start if not set. Basically just took over the 12 byte padding we were using for the hash.
• supports specifying command to use with --bw-command
• Basically ignores rpc and eval commands since this have another argument expression to consider. Its probably easy enough, I just ran of to time for testing this

Next PR I want to remove the add_start_script and move to C. I also think I want to scope bakeware options in the release to a :bakeware key so we don't conflict with any other options

[fhunleth]

@jjcarstens I rebased this and made a couple minor updates to enforce the 12-char command limitation. It's not ideal, but it should be good enough for the start script.