add a few extra tests

I've added a few test for what follows. I'm having trouble testing the inline ones, because IO.ANSI.escape_fragment doesn't seem to return the leading escape sequence when testing. 


Anyway:

1. \*, \**, \_ and so on are only considered delimiters if they are not followed by space:

  I believe the code already does this. Do you have an example where it doesn't?

2. Similar, a list is only marked as so if * is followed by space;

  I think this is already there, too. 

3. The current implementation doesn't work with nested lists. Here is the expected result:

    iex(10)> Markdown.to_html "* foo\n  * bar"
    "<ul>\n<li>foo\n\n<ul>\n<li>bar</li>\n</ul></li>\n</ul>\n"

  That's a bug. I'll fix it

4. A paragraph is terminated when there are two new lines and any number of spaces in between those lines. We don't consider the spaces. I saw that when a list was not showing up:

  Do you have an example of this? It seems to work here.

5. Discussion: should we align the markdown paragraphs by default as we do here? That's because heredocs already do that in Elixir:

    """
    foo
    """

    #=> "foo\n"

Elixir takes into account the closing """. If someone writes docs like this:

    """
        foo
    """

 I'd rather say they should fix their docs then silently accepting it (because other tools like ExDoc won't silently accept it).


That's easy to remove if you want.