Added math tag support, fixed whitespaces management

Nerdz Grammar:
* `[math]`: block level math tag (`$$ latex math $$`)
* `[m]`: inline level math tag (`$$$ latex math $$$`)
* Fixed spoiler regexp that was causing an additional blank line before closing tag

General:
* Now "consecutive" text-only paragraphs are merged together and the blank lines between them are preserved
* Removed trailing spaces/blank lines at the end of the output

Layout for the output of block tags is now (more kawaii :3):

    [opening_tag]
    content
    [closing_tag]