Kiss-headers

*Changes :*
- **Bugfix:** Disallow single colon in username when using BasicAuth.
- **Add:** Create `ContentSecurityPolicy` custom header in builder.py.
- **Improvement:** Add ready-to-use methods in `Authorization`, `BasicAuthorization`, `AltSvc`, `StrictTransportSecurity`, `ContentSecurityPolicy`, `WwwAuthenticate` and `ContentRange`.
- **Improvement:** Adjust/Add get_qualifier() method when needed in builder.py.

*Changes :*
- **Feature :** Polymorphism, transform a Header object to its corresponding subclass. 26
- **Feature :** Implement pop() and popitem() methods in Headers.
- **Bugfix :** Discard multiple entries detection if the header is 'Subject'.
- **Bugfix :** Acknowledge the folding LF + 1 space technique to unfold.
- **Add :** Cookie custom header class in builder.py.
- **Bugfix :** Fix how WwwAuthenticate custom header is built.
- **Feature:** Add unfolded_content property to Header class.
- **Other :** Provide scripts for developers. 24

*Changes :* 22 21 20
- **Feature :** Retrieve comments in header content using newly created `comments` property on Header.
- **Feature :** Parse headers that are UTF-8 encoded when using `parse_it()` function with **bytes**.
- **Feature :** Attribute value in header are unfolded by default when retrieving it. `content` property is not altered.
- **Other :** Minor code simplification in Header class. Remove redundant private member.

*Changes :*
- **Feature :** Add method `has_many()` to verify if an header or attribute name has multiple entries associated.
- **Improvement :** Allow class that inherit from `CustomHeader` to override their header name.
- **Feature :** Add `BasicAuthorization` in the ready-to-use headers objects.
- **Bugfix :** Implement bool method for `Headers` class to act like list() does.

*Changes :*
- **Bugfix :** Better detection of an illegal header name.
- **Improvement :** parse_it() was too permissive and allowed illegal header name to be included.
- **Feature :** Reversed can now be called upon an Headers instance.
- **Feature :** Sorted can be used with an Headers instance to return a list of Header in alphabetical order.
- **Bugfix :** ContentDisposition custom class was not encoding fallback filename properly.
- **Improvement :** Headers class can be instantiated like `Headers(Header("Content-Type", "text/html"), Header("Allow", "POST"))`.

*Changes :*
- **Bugfix :** Support for (yet unreleased) Python 3.9
- **Other :** Disallow illegal header name when using `Header` class.

Also a few improvements in the code struct. 👍