render: Support custom formatters

[abhinav]

The renderer currently unconditionally reformats Go code samples, and only Go code samples.

There are a couple issues with this:

• this can lead to inconsistent formatting if some samples are fully valid Go code and others aren't
• when desirable, this only works for Go, and nothing else

This proposes a new WithCodeFormatters option for the renderer. If used, this option replaces the list of code formatters used by the renderer.

Code formatters are matched by language, optionally with aliases. By default, only a formatter for "go" (with the alias "Go") is included -- this matches the prior behavior.

If WithCodeFormatters is called without any arguments, it disables code formatting completely.

[karelbilek]

To quickly explain - the original usage for this was mainly for go samples in code documentation.

I think the original behaviour doesn't need to be enabled by default... I think default behaviour should be to just not do any reformatting.

(I am surprised that Uber is using our stuff, but also happy! :D )

Btw maybe some added tests would be nice...

[abhinav]

I think the original behaviour doesn't need to be enabled by default... I think default behaviour should be to just not do any reformatting.

I can make that change if you both agree that that's desirable. I retained the original behavior mostly to keep this change backwards compatible.