Ignore generated files in .NET

Summary: Ignores files marked with `<auto-generated>` which is the pattern in .NET. While this will unfortunately not show as generated in Differential (long story), they will be ignored from linting. This will eventually be extended to support Differential ignorance (Phabricator needs some modifications).

Test Plan: Touch some generated files, notice that linters like the long line linter avoid them.

Reviewers: kspearrin, jmillard

Reviewed By: jmillard

Differential Revision: https://devportal.imobile3.com/D3861