How to specify multiple patterns to process for a source string.

[guyboertje]

To prevent requiring the user to use conditional branches we should be able to specify multiple patterns to apply to the same source field.

Currently the spec is silent on the number of patterns per field source and only deals with the specification of the prefixes and suffixes of the field definition itself but not the broader scope of functionality.

Prior discussion: logstash-plugins/logstash-filter-dissect#56