Add 'attribute' option to scrape tool

The idea is to provide an "attribute" option to scrape tag's attribute value (not tag content itself). 
<h3>Use case</h3>
With the many frontend libraries available now, we often encounter data embedded in a tag attribute, let's take the following test.html example:
```
<section>
	<h1>Title</h1>
	<div>
		<div class='interesting' data-entries="{&quot;entries&quot;:[{&quot;id&quot;:1,&quot;name&quot;:&quot;thefirst&quot;},{&quot;id&quot;:2,&quot;name&quot;:&quot;thesecond&quot;}]}">
			<span>Some interesting content</span>
		</div>
		<div class='interesting'>
			<span>Some other interesting content</span>
		</div>
		<div class='interesting' data-entries="{&quot;entries&quot;:[{&quot;id&quot;:8,&quot;name&quot;:&quot;thetenth&quot;}]}">
			<span>Some interesting content</span>
		</div>

	</div>
</section>
```
to extract the "data-entries" attribute value from the matching tags (having the specified attribute) , we can then use:
```
$ scrape -e ".interesting" -a "data-entries" test.html 
{"entries":[{"id":1,"name":"thefirst"},{"id":2,"name":"thesecond"}]}
{"entries":[{"id":8,"name":"thetenth"}]}
```