Code eval: use show on last eval

[tlienart]

Would be good to have it such that

```julia:ex
a = 5
```
\output{ex}

shows 5 instead of having to slap a @show a at the end for this.

[tlienart]

This is now partially supported, the showall local page var does this for every code block.

To be complete, we should add a \show command which acts as \output except that it shows the result (not stdout).