Allow lints to be customized

[johnpryan]

Certain lints like prefer_const_constructors might not be desirable for codelabs aimed at beginners. See this discussion for more context.

It would be great if authors could include an analysis_options.yaml file in their Gist or workshop, rather than using // ignore_for_file: syntax.

[domesticmouse]

Given that customised lints would require stopping and re-starting the analysis server, this change would balloon the required number of servers by the number of seconds the restart takes (we usually serve most requests in under a second).

I suspect this will fall into the DartPad isn't an IDE bucket.

[parlough]

I was thinking about this and another solution for some of the issues would be to allow for pad creators to somehow specify and hide a header to the file(s). For workshops this could be applied to all steps. This way they could optionally ignore specific rules or all lint rules.