fix #12: Allow writer to specify an identifier to a given title

Source
```
=title1
=title2=
=title3=newtitle
=title4\=nonewtitle
=title5=new title
```

Generated
```html
<h1 id="title1">title1</h1>
<h1 id="title2">title2</h1>
<h1 id="newtitle">title3</h1>
<h1 id="title4-nonewtitle">title4=nonewtitle</h1>
<h1 id="new-title">title5</h1>
```