Allow custom name for the datasource template

[kovaxur]

Changed the datasource linter to allow any name for the datasource template using regexp.

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[rgeyer]

Hello @kovaxur, can you look at the implementation merged in #124 and see if it solves this problem for you?

The issue of allowing multiple data sources has been discussed, and #124 was the solution which got group consensus. While it would not allow any named variable for the data source, it will allow multiple, and tries to be reasonable about naming by enforcing consistency.

[kovaxur]

Yes, it's fine thanks.