Support converting to Java types from multiple string values

[rvesse]

Currently Airline maps each argument passed in to a single Java type. In some cases it would be nice for multiple string values to be combined into a single type, this would make it possible to instantiate much more complex types than is currently possible.

Doing this would require changes to the following:

• Need to be able to annotate that an option/argument should convert values in batches
  • If we allow batch sizes that are not equal to the arity also need to annotate that
• Option and argument parsing needs to take the conversion batch size into account
• Type converters need to change their signature to accommodate this