Add support for HTML5 and custom definitions

[LukeTowers]

This implements the elements and attributes for HTML5 defined in https://github.com/kennberg/php-htmlpurfier-html5, and also adds support for defining a custom definition along with adding custom elements and attributes to that definition.

It is fully backwards compatible, existing configurations that don't have the new config values will still work correctly, they just won't see the new definitions until they update their configurations (which they'd need to do anyways to set new properties for the allowable).

This PR correctly implements the desired functionality that was added in #26 by @CodeL and then removed in #30 by @itbdw because it was broken when users did not update their configurations.

@mewebstudio If you are not willing to continue on with support for this project, then please let me know and I'll be happy to continue it on in my fork on the project.

[0x5459]

@LukeTowers
Why "video" tag still filtered?

code:
var_dump(clean('<video></video>'));
output:
string(0) ""

[LukeTowers]

@ty666 I'm thinking something changed in the underlying library that may have caused issues with customizing it as is done in this PR.

[zarianec]

It looks like HTMLPurifier_Config::inherit($config) that used when creating purifier instance (code -> code), doesn't use custom definitions from parent config object.

I'm not sure why HTMLPurifier_Config::inherit() used and is it possible to avoid using it safely.

Updated:
See #80