Support for boolean modifiers

**Warning:** major changes — shortcut matching replaced by wildcard matching (`*`).

Input:
```javascript
{
    block: 'button',
    mods: {
        theme: 'normal',
        disabled: true
    }
}
```
Matching:
```javascript
bh.match('button_theme', function(ctx) {
    // Will not be applied anymore
    ctx.bem(false);
});
bh.match('button_theme_*', function(ctx) {
    ctx.tag('button');
});
bh.match('button_disabled', function(ctx) {
    ctx.attr('disabled', 'disabled');
});
```
Result:
```html
<button class="button button_theme_normal button_disabled" disabled="disabled"></button>
```