:describe and :it can't handle double-quoted strings

The question is whether or not it should be risked giving the user the power to use double-quoted strings in the argument to `:describe` and `:it`.

Currently the single quotes are hard-coded. `:describe` and `:it` only accept single-quoted strings, not any Vim string. So I think at the least these statements in the documentation should be changed:

    {subject} is a string to describe the subject of the examples
    {example} is a string to describe the example

Personally, I think double-quoted strings should be allowed. Some users might need, for example, a Unicode escape sequence or a tab character.

I have attached a patch.

I realize that this might make compiling less robust. Especially for `:it`, where the `{subject}` is used as a function name, it might be dangerous to allow escape sequences. The decision is up to you.