Backslash escapes not being honored

[davidearl]

In principle you should be able to write ![alt](http://example.com "this is a \"title\"") (see https://daringfireball.net/projects/markdown/syntax#misc ) but this fails the pattern recognition completely so the above appears verbatim in the html.

Note that the resultant double quotes must be escaped within the html generated <img ... title="this is a &quot;title&quot;"> (see issue 25)