tweaks to echo commands with output, inline values

These aren't the prettiest things, but this changes weave.jl to:

* handle inline expressions like `x`{.julia}

* interweaves cmd/results like knitr

* I can't seem to get something like this to plot, though the JavaScript gets inserted. What calls the `draw` function and how does the parent_id get inserted into a document?

```
~~~{.julia}
using Gadfly, Compose
p = plot(sin, 0, pi)
draw(D3(6inch, 6inch), p)
~~~
```

Hope something like this can be added into weave.