Add Data::add method

[Palladinium]

Data is essentially a thin wrapper around a Vec<Command> with nice syntactic sugar, but there doesn't seem to be a way to pass Commands directly into the buffer, and sometimes the syntactic sugar isn't the easiest way to go about things. This PR adds just that.

[IvanUkhov]

Thank you! Can you please give an example where this method would be preferred?

[Palladinium]

In my use case, I'm procedurally generating SVG paths, and I have a bunch of structs that construct path segments through Into<Command>.

Without this PR, my options are:

• match over the returned command just to call the corresponding syntactic sugar method on Data, which looks like an anti-pattern since the method just constructs another identical Command
• Change the structs to take a Data and mutate it, which makes it harder to chain nicely
• Convert back and forth between Data and Vec<Command> to intermix the syntactic sugar methods and pushing generic Commands to the Vec

With this PR, I can write code like:

Data::default()
  .move_to((10.0, 0.0))
  .add(Thing.into())
  .move_to((30.0, 0.0))
  .add(Thing.into())
  .close()

Which matches the way Document::add() looks.

[IvanUkhov]

Good points! Thank you.