Feature request: Reorder / sort fields

[raphCode]

I like to have some field of my bibliography entries in a certain order, for example:

• type
• title
• author

I would like to see a command line flag to specify the order of certain keys, which is then obeyed when printing the hayagriva bibliography. Unspecified keys are output afterwards, either unsorted or with an internally-defined sort order (to guarantee stable ordering with future changes).