Make `noms struct set` and friends take an optional equals operator separating key/value pairs

[aboodman]

I received feedback that a command like:

noms struct new /tmp/mydb f1 v1 f2 v2

is confusing to look at, and putting equals signs in there would clarify it, like:

noms struct new /tmp/mydb f1=v1 f2=v2

This seems reasonable to me, but I don't want to have to type the equals signs all the time once I know the command. So I'm thinking maybe we just make them optional.