Added link support for github imports

[nanjj]

So that for svg output the link will be generated, for we can generate map file:

dot -Tcmapx -o file.cmapx file.dot

Using svg file in html:

<object type="image/svg+xml" data="file.svg"/>

Using png file in html:

<img src="file.png" usemap="#godep"/>
<map id="godep" name="godep">
<area shape="poly" id="node1"...

[kisielk]

Good idea. Can you update the tests as well?

[nanjj]

@kisielk Added unit tests.