Flexible Param input

[bovee]

A lot of parser states are parameterized. For example, both #12 and #13 require a file name so we can open another ReadBuffer to store in the state, TSV/CSV parsers can take delimiters and number of header lines, and kmer-parsers need a k.

Right now, we pass initial params into the state as a "P" object scoped to each state which we also require to have a Default so that we can use None across parsers. An alternate design could take an Into<P> object where P: Default + From<Params> for convenience (where Params is some kind of { filename: String, other_params: BTreeMap<&str, Value> }?)

[bovee]

I ended up extending get_reader to support additional params that must be used (except for a filename param that gets inserted by default) in 5f72d0468c8c8d343370638020a9334b31e5d704.

Some alternate designs that didn't work:

• Extending StateMetadata to add a method that would inject the params. This would require instantiating a State first, which generally requires some amount of parsing and since the params wouldn't be injected yet, the parser couldn't use them.
• Making a generic Params object that all State parsers would need to accept. This wouldn't allow parsers to use State fields to move data from the parse phase to the get phase unless the Params object also had a generic param field which made the design pretty complicated.