* FEATURE Add gfm-stylesheet

Add a GitHub Flavored Markdown Stylesheet,
with the keyword »gfm«.

* BUGFIX Don't use example title as fallback
* BUGFIX Don't use example stylesheet as fallback
* FEATURE !! Enable method chaining

Refactor all methods to enable fluent interfaces.

An example title is always set as fallback.
This is unexpected behaviour and not desired for most use cases.

Use a neutral fallback instead if no title was given.

An example stylesheet URL is always set as fallback.
This is unexpected behaviour and not desired for most use cases.

Don't render a stylesheet if setContent() was used and setStylesheet()
was not used explicitly.

* FEATURE Docs: Replace screenshot
* FEATURE Add static method to fetch example template
* FEATURE Add minimal tests

Use a PHP code example for the screenshot

Add a static method to get the example template.

- No need to create the object
- Preparation to solve the bug that example styles
  are set by default

* FEATURE Escape special chars in title and attribute options

Markup should be rendered as raw HTML of course, but all other
strings in tag attributes like »stylesheet url« or »title«
are escaped by default to prevent broken/invalid HTML.

Add link to html5example.com single-serving site.

* FEATURE Docs: Be more verbose in options description
* FEATURE Docs: Add example usage case
* BUGFIX CGL: Correct PhpDocs
* BUGFIX Styles: Rename ambiguous keyword
* FEATURE Enable setting additional metadata

It may be neccessary to add additional metadata like a
canonical link. Add an option to support this use case.

Using »mini.css« should result in a valid relative file link,
but generates a link to the CSS framework instead, since this
is a reserved keyword as well. Rename this keyword too avoid
this unwanted behavior.

Replace wrongly described return types or
attributes in PhpDoc comments.

* FEATURE Apply coding guidelines

Add some awesome README badges

Add a minimal CI script (lint fatal errors & code style)

* FEATURE Cache inline stylesheet

Avoid requesting the CSS file on every load and try to
cache it instead with a minimal caching mechanism.

* FEATURE Add option to render stylesheet inline
* FEATURE Use headline as title fallback

Make stylesheet render mode more verbose

Add an option to render the given stylesheet inline (using script tags)
to avoid an external request due to privacy concerns.

If no title is set, then parse the document and
grab the first H1 as title fallback.

This way this method call may be axed in most situations.

* BUGFIX CSS CDN needs specific version for skeleton

Skeleton cant use the version fallback of the CSS CDN,
because of e changed file structure and therefore needs
a specific version constraint.

Add CDN links to several more minimal CSS frameworks.