Add group function

[bdarcus]

Create three functions.

First, make_group_key:

Take a vector StyleGroupKey and concatenate the values returned from the respective field data using string_for_key.

Second, string_for_key:

When given a key return the string value from ProcReference::data.

Third, group_proc_references:

Group the vector returned by Processor::get_proc_references and add a ProcHint to each, consisting of group_index, group_length, and group_key; return a new vector of ProcReference.

Finally, add a test to processor_test.rs to confirm correct ProcHint::group_index, values using the examples/bibliography.yaml.

[bdarcus]

Closed via #17