Provide configuration mechanism

[blackwinter]

• Allow Fixes to configure the entity marker and the - possibly mandatory - version (cf. Metamorph: entityMarker = the character used to separate entity names, version = the Metamorph (here: Metafix) version the document is intended to work with).
• Could be implemented as a comment/preamble (first line with specific syntax) or as Fix function(s) (there doesn't seem to be a corresponding mechanism in Catmandu).

[fsteeg]

Also for array marker & name (cf. JsonDecoder.setArrayMarker() and JsonDecoder.setArrayName() in metafacture-core), see https://github.com/metafacture/metafacture-fix/pull/60#discussion_r738384778 & https://github.com/metafacture/metafacture-fix/pull/60#discussion_r738385173.