--meta-tags option can now accept either a path to file or directory.
And one can use the option multiple times to name several files
one-by-one.

More accurate, as it's not only Markdown. If one desires doing
Markdown-only, then he can include RDiscount by himself and do
whatever he wants.

Previously it was either builtin ones or user-define ones.
Now one can have a combination of the two.

Each meta-tag class is now responsible for rendering a block of all
meta-tags of same type, complete with title and everything.
The transform method has been replaced with to_html method that
converts array of meta-tag contents into output HTML. No more are
there @title and @hidden attributes.

When there's more than one file, pop up a menu with all files.

One can now write:

    @param {Object} [scope=this]

And the default value will be `this`.

Instead printing out a warning.

Avoid fatal crash - instead print out warning.