Fix some pep8 format with flake8 and support new feature.

[keysona]

Some block comments:

#---xx

to

# ---xx

And some errors：trailing semicolon in python statement.
Like this：

_bq_one_level_re_spoiler = re.compile('^[ \t]*>[ \t]*?![ \t]?', re.M);

New feature:
A link with "_blank" in target attribute.
For example :

[Ref](http://www.example.com)

To

<p><a target="_blank" href="http://www.example.com">Ref</a></p>

extras option is "link-with-blank".

[keysona]

#213

[nicholasserra]

Thanks!

So the idea here is that when you turn on the link-with-blank extra, any links in markdown will have target="_blank"?

[keysona]

@nicholasserra

Yes, like nofollow.

Maybe this extras option name link-with-blank is too long?

I think it can be shortest, maybe blank or _blank?

[nicholasserra]

Gotcha, cool. I don't think it's too long, but I think it's hard to understand. How about target-blank-links?

[keysona]

@nicholasserra
Good name!
I agree to use target-blank-links, it really easy to understand!
Now I change the name:-).

[nicholasserra]

Thanks!

[keysona]

#213