Drop langtags on offset text but support on paras

Now that language attributes can be arbitrarily added to any inline,
  a special syntax for them doesn’t make much sense.
However, that syntax would be useful on block‐level elements.

There isn¦t a real technical reason for the explicit `$´ end of the
  language tag, since the following whitespace could have been used to
  end it also, but it makes the syntax a bit more obvious, matches the
  existing format with the shebangs, and is less likely to conflict
  with i·d syntaxes (which might include `@´ but probably won¦t also
  end in a `$´).

You can still make i·d¦s like `@es$´ if you need by explicitly
  language‐tagging: `¶@es$@en$´.

This commit also allows block sigils to not be followed by a space if
  they are immediately followed by a pilcrow.