test and fix markdown parseLink

[MartinNowak]

No description provided.

[s-ludwig]

Thanks. Looks good to merge.

[MartinNowak]

My actual problem was that I switched from sundown which allowed spaces in URLs.
But as this isn't broadly supported I left it as it was.
There is also no test case for this in https://github.com/jgm/peg-markdown/tree/master/MarkdownTest_1.0.3.
Would be interesting to use the test suite though.

[s-ludwig]

My guess is that this test suite will get particularly interesting when whitespace gets involved. Getting together a half clean markdown parser that is not based on a cascade of regexes was an interesting challenge, but so far I haven't looked at all at how line breaks and other whitespace is output (at least only so far that the rendered HTML is correct). But I agree that this should be the goal. In fact this module screams for unittests (but was still done in a time when getting things done fast had priority over writing unit tests for already working things).

I'd also really like to support Michel Fortin's extras and SmartyPants on top at some point. But before that is tackled, a proper test suite is mandatory, too.