Helper to check property existence in ctx and in this

Something like:

``` js
block('b42').match(function() { return this.ctx.icon; }).tag()('b')
// →
block('b42').has('icon').tag()('img')
```

and maybe like (or params/guards?):

``` js
block('b43').mod('flagged', 'yes')(function() {
this._icon = true;
return applyNext();
});
block('b42').match(function() { return this._icon; }).tag()('b')
// →
block('b43').mod('flagged', 'yes')(node => (node.flag('icon', true), applyNext()));
block('b42').hasFlag('icon').tag()('img');
```

And if we need negation (in addition to #246):

``` js
block('b42').match(function() { return !this.ctx.content; }).content()('default content');
block('b42').not(has('content')).content()('default content');
```

Start by reviewing the existing match(), mod(), ctx, and flag behavior described in the examples, along with the related issue #246. Done would require an agreed helper syntax and semantics for checking properties or flags on ctx and this, including the proposed negation form.

Written by the indexing model from the issue text.