Implement custom value mapping

Sometimes we would like to change an value encountered at the source to a different value to be stored in the MP. Ideally, the mapping from source value to target value should be easily injectable into the ingestion pipeline.

If this concerns only a small number of values, this may be still expressed in the JOLT-configuration file, however the mechanism is quite cumbersome and would make it difficult to adjust dynamically. Thus, there is a suggestion to implement a custom value mapping mechanism, which would take an external csv-file (either next to the JOLT configuration or on a different git-repository) in the form:

source_value	target_value
DH	Digital Humanities
Digit. Scholarsh. Humanit.	Digital Scholarship Humanities

This could be ideally implemented as a custom API-method that can be called from the JOLT-configuration.

(notify: @lbarbot, @klaus.illmayer, @anowak )