Add the ability to customize the example_text function

[aerostitch]

Hi!

When you generate some flavored text like Markdown it would be really nice to reuse directly the builtin functions instead of having to duplicate chunks of code like I had to do in davecheney/godoc2md#20
This PR adds an example_test2 function that supports a custom title for the example and custom elements to put around the code block (so we can add " go" and "")
Let me know what you think about it.

Thanks
Joseph

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Message from Brad Fitzpatrick:

Patch Set 1:

Proposals should go via https://golang.org/s/proposal and not via new code.

Please open a proposal if you wish to add new functionality.

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Message from Joseph Herlant:

Patch Set 1:

Hi Brad,

Sorry for that.
The proposal has been created: golang/go#24862

Thanks
Joseph

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