conditional attributes!

This is something that's come up time and time again - at least as far back as #235 and most recently with #1211 - the ability to do this sort of thing:

```html
<div {{#if foo}}class="bar"{{/if}}>...</div>
<div {{attributeList}}>...</div>
<div {{>somePartial}}></div>
<div {{#each attrs}}{{name}}="{{value}}"{{/each}}></div>
```

(I'm not saying these are all necessarily good ideas; just that they're all now possible. The syntax highlighting rules of your editor will largely determine whether you can stomach it - luckily Sublime Text does a saner job that GitHub, it seems.)

It's long overdue! The mechanism is actually fairly straightforward, and it should work with just about any mustache/data, of whatever complexity - though I've learned better than to immediately merge in the result of late-night coding sessions. So I'll leave this open for the moment, until I'm happy that I haven't broken anything.

If you want to have a go in the meantime, these are the steps:

* clone the repo and `cd` into it
* `npm install` the dependencies
* `gobble` to build the library (needs [gobble-cli](https://github.com/gobblejs/gobble-cli) installed globally)
* play around with the contents of the `sandbox` folder
* `gobble build some-folder` to build the library to `some-folder`
@Rich-Harris Getting an error in Safari. Is this supposed to be `node.namespaceURI === namespaces.svg`?