Bugfix and stuff

* Added 2 new functions to avoid content of specified bbcode tags to be parsed: `_HashBBCodeBlock()` and `_HashBBCodeSpan()`. The former is used for BBCode which is represented as blocks (such as `[code]` and `[math]`), the latter is used for inline BBCode (like `[m]` and `[url]`).
* Removed extensions that were used to parse content of some BBCode tags
* Ordered lists are now available (types: number, letters and roman letters); there is also a starting index auto-recognition for ordered lists
* Horizontal rules are now directly converted into BBCode (before it was Markdown > HTML > BBCode)
* Removed generation of paragraphs (`<p>`)

There is a bug that has to be solved: if a BBCode block is followed by `_`, all the Markdown code until the next new line is not parsed.