Add AutoDoc command for immediate code blocks

[gwhitney]

Unlike with all of the other @Begin.. / @End.. pairs, the @BeginCode / @EndCode facility does not have a variant which inserts the quoted code at the current point in the manual; one has to issue an @InsertCode command explicitly for the code to appear. I suggest it would be useful to have an immediate version.

I have proposed code that includes this change which I will post soon.

[fingolfin]

This has been resolved via PR #311 which introduced Markdown-style fenced code blocks, which IMHO solves this more elegantly (and in a way that is intuitive for people who know Markdown)