Allow searching for substring in content

[BTWS2]

has_content doesn't allow to check if a substring is present. Either add contains_content or add substring search functionality to has_content.

Use case: Make exercises more personal and detecting plagiarism easier. Structure has to be there, but content can be different for each student e.g.

"Make mini website with an image, a paragraph of at least 50 characters and a table of 3 rows. If the image can't be displayed the text should contain the text "not found".

• Add option to compare only plain text, removing all tags (also child tags). See example below.
• If possible option with direct to only search in top level. See example below.
• Add option to specify minimum or maximum characters the plain text tag should contain.

<ol>
    <li>Fruitmand
        <ul>
            <li>Appel</li>
            <li>Peer</li>
        </ul>
    </li>
    <li><a href="groenten.html">Groentenschaal</a></li>
</ol>

html = HtmlSuite(content)
body = html.element('body')
ol = body.get_child('ol', 0)
li0 = ol.get_child('li', 0)

html.make_item("Het eerste lijstelement bevat de tekst 'Fruit' in het topniveau.",
        li0.contains_content("fruit", case_insensitive=True, remove_tags=True, direct=True))  # True
html.make_item("Het eerste lijstelement bevat de tekst 'Peer' in het topniveau.",
        li0.contains_content("peer", case_insensitive=True, remove_tags=True, direct=True))  # False
html.make_item("Het eerste lijstelement bevat ergens de tekst 'Peer'.",
        li0.contains_content("peer", case_insensitive=True, remove_tags=True, direct=False))  # True