[MINOR] pattern: add support for argument parsers for converters

[MINOR] pattern: add support for argument parsers for converters

Some converters will need one or several arguments. It's not possible
to write a simple generic parser for that, so let's add the ability
for each converter to support its own argument parser, and call it
to get the arguments when it's specified. If unspecified, the arguments
are passed unmodified as string+len.