Make VC helper fields configurable

Adds KALAMAR_VC_HELPER_FIELDS environment variable and vc_helper_fields
configuration option to add or remove fields from the virtual corpus
builder at runtime.

Syntax: KALAMAR_VC_HELPER_FIELDS=+award:text,-docTitle
  +name:type  adds a field
  -name       removes a field

The result is always sorted alphabetically. Analogous to
KALAMAR_HINT_FOUNDRIES (aa6709c).

Required for DeLiKo@DNB

Change-Id: If9ad570133f78515aa6ed0ef87424c0843e76d9c
diff --git a/kalamar.conf b/kalamar.conf
index 4682c0a..e0ff62c 100644
--- a/kalamar.conf
+++ b/kalamar.conf
@@ -84,6 +84,13 @@
     ## Can also be set via KALAMAR_HINT_FOUNDRIES environment variable.
     ## Use '-foundry' to exclude from defaults, e.g. ['-spacy', '-corenlp']
     # hint_foundries => ['base', 'corenlp', 'dereko', 'malt',
-    #                    'marmot', 'opennlp', 'spacy', 'tt']
+    #                    'marmot', 'opennlp', 'spacy', 'tt'],
+
+    ## VC helper fields
+    ## Controls which fields are shown in the virtual corpus builder.
+    ## Can also be set via KALAMAR_VC_HELPER_FIELDS environment variable.
+    ## Use '+name:type' to add, '-name' to remove from defaults.
+    ## Result is sorted alphabetically.
+    # vc_helper_fields => ['+award:text', '-docTitle']
   }
 }