shell file builder module for jenkins-jobs-builder

Command list for shell builder tend often to become very large scripts.

Every time a new version of the script needs to be copied into the jjb yaml definition.
- syntax highlighting on this large scripts is unavailable.
- the definition itself becomes less readable.
- input sanitization is required for shell variables surrounded by curly braces for the use in jjb

I did not find any jenkins plugin that executes an "attached" shell script file and standard yaml does not provide an "include" statement, so it could be useful to implement in jjb a shell script file module that imports and sanitize a file containing the command list during the XMLification.

Inclusion should be trivial; sanitization and the possibility to specify a relative path for the script file not so much