this should break urls that attempt to include a protocol, or port (these are absolute URLs and should have a whitelisted protocol for use)
but URLs that are relative, or relative from the site root should be preserved (though characters non essential for the URL structure may be urlencoded)
this approach has significant advantages over attempting to locate something like `javascript:alert(1)` or `javascript:alert(1)` (which are both valid) because browsers have been known to ignore ridiculous characters when encountered (meaning something like `jav\ta\0\0script:alert(1)` would be xss :( ). Instead of trying to chase down a way to interpret a URL to decide whether there is a protocol, this approach ensures that two essential characters needed to achieve a colon are encoded `:` (obviously) and `;` (from `:`). If these characters appear in a relative URL then they are equivalent to their URL encoded form and so this change will be non breaking for that case.
Fixeserusev/parsedown-extra#67.
This introduces PHP 5.3+ late static binding to the Singleton pattern in Parsedown.
It will return an instance of Parsedown which inherits the class which
called the `instance()` method rather than always returning instance of just `Parsedown`.
Tests are testing this feature with a test class which inherits from Parsedown.
Notice that calling `instance()` with the default arguments after an instance of
`Parsedown` was already created, it will return it even though it is from just
an instance of `Parsedown`. So this is fixing the problem just partially.
