features needed for Go snippet insertion

[jba]

My application is inserting snippets from compilable Go code into a README.md. I wasn't able to use embedmd for a couple of reasons.

• I want to exclude the matching regexps, so I can write comments in the Go code that delimit the snippet. I don't want to match the snippet code itself, because that is fragile—it can change. See support a flag to exclude matching lines #46.

• I really want to exclude the line the regexp is on. Not a big deal, just add .* to the regexp, but I need to do that for every one, so it's a little error-prone.

• The snippets are typically inside functions, so they have a leading tab. I need to remove this tab from each line so that my Markdown code blocks are not indented. No way to do this in the current embedmd model.

• Another whitespace issue: at top-level, gofmt puts a blank line between code and a following comment. If that comment is an end-of-snippet delimiter, I need to drop that final blank line, or it will end up in my markdown. (I want to preserve blank lines inside the snippet, however.)

• Sometimes one wants to write ... in a snippet as a placeholder. E.g.

  tokenSource := ...
  client = New(ctx, tokenSource)

  Since that isn't legal Go, I need something that will transform legal Go into it. E.g. replace ELLIPSIS and _ = ELLIPSIS with ....