Adding directives to selectively disable comment parsing.

This change adds so-called 'directives' to our comments.  A directive is
a comment line whose only contents are a hash, followed by the directive
name, e.g.

```
;; #DirectiveName
```

Directives are not included in the `*comments*` vector in any case.

The two directives implemented here are "#MargDisable" and
"#MargEnable".  The former, when encountered, causes comments read by
`read-comment` to be ignored.  The latter re-enables it.

The rationale for this change is to accommodate e.g. license-header
boilerplate without polluting the generated docs.
Replacing "#" with "@" as the directive-start character.

In the excitement of getting something working and polished, I
overlooked the fact that "#" is indeed part of Markdown syntax, and that
there is little to distinguish `;; #Directive` from `;; # Title`.

"@" is visually distinctive, and is not currently part of Markdown.
Merge pull request #126 from benjamin-bader/add-comment-directives

Adding directives to selectively disable comment parsing.