add scale macro to template

[floswald]

with this macro, as per gnab/remark#72, one can now do

# A plot

```@example index
using Plots; gr()
Plots.plot(rand(10))
savefig("statplot.png");
```
![:scale 90%](statplot.png)

this solves my issue #9 at least somehow (it's not automatic, but it works).

[sffc]

How about naming the macro img and keeping the alt text?

remark.macros.img = function (altText, width) {
  var url = this;
  return '<img alt="' + altText + '" src="' + url + '" style="width: ' + width + '" />';
};

Are there any other options we might want to support? Adding a macro like this to the main library means that it will be hard in the future to change it.

[piever]

Somehow I'm not sure we should add some specific macros here: if they didn't make it to Remark.js, then it's just confusing to have them here. OTOH it could be nice to add a way to add custom macros (either as a text file or a Julia string: I would tend to prefer the text file as there are no problems in escaping and it's easier for the user to write some normal javascript in a .js file with the highlighting provided by the editor).

So maybe the easiest is to allow the user to pass a filepath to macros keyword which would be used to define macros. Then this macro here (or maybe both) would go in the README as examples.

[tbeason]

Possible to revisit this?

[piever]

How do things work with plain Remark.js? Is the scale thing supported? If it is, just rebuilding Remark.jl should download the latest Remark.js and give support of the feature for free.

[tbeason]

Sorry, I meant the idea to include custom macros in general. I agree with what you said above about perhaps just somehow inserting an external file into the index as a good way to support it.