Undocumented Pipeline yaml functionality

A best practice across all programming disciplines is to avoid repeating identical lines or values when writing code. In my case, I have 3 tables with identical columns but differing data. The "import with csv" action looks *almost* the same for each table.  This yaml syntax allows me to write the value for `header_row:` once and define a reference to that value in-line. Any time I need that same value, I can use the reference instead of the actual value.

Keep in mind this is a purely textual modification to the yaml, and after the yaml is loaded, the resulting values are hard coded again.  After loading a pipeline with yaml references, you could edit the pipeline in the UI to have different `header_row` values.

I am using yaml references to define the fields i import, however this simple reference is insufficient to create a valid "import with csv" block.  Each field must also have mapping in the form of `qb_field_for_a` then `qb_field_for_really`, then `qb_field_for_long`... one for each field listed in `header_row`.  The value of the mapping is the field ID for the destination table.  For me, the destination field ID is not consistent, so I can't simply repeat all the `qb_field_for` using yaml references, because the values differ.

Honestly, it's not clear how useful it is, but I figured someone might get some use.