Allow additional selectors for meta parsing

[MaxDall]

This adds a new parameter additional_selectors to get_meta_content. The parameter expects a mapping in the form of a dictionary to add additional selectors to the meta parser.

Let's assume the following meta tag:

<meta itemprop="headline" content="The Gospel According to Paul: A Comprehensive Study of Paul's Message in the 'New Testament'">

One can now pass the following mapping to the parser to extract this malformed meta tag.

{
    "itemprop": CSSSelector("meta[itemprop]")
}