add support for multiple templates in config

[holger-waschke]

for better clearness we use multiple templates, e.g. one template per jira custom field. as we use templates for mapping they get quite big and its handy to separate them. in additation added a split template handler

template:
  - jiralert_psk_key.tmpl
  - jiralert_insight_ia_key.tmpl
  - jiralert_Entwicklung.tmpl