Crashes on links that only have # as value for href

[michaellopez]

Basically this:

<a href="#">Crash it!</a>

will crash the build:

Pushstate server started on port 2999

🕷   Starting crawling http://localhost:2999
TypeError: Parameter "url" must be a string, not object
    at Url.parse (url.js:87:11)
    at urlParse (url.js:81:5)
    at Url.resolve (url.js:641:29)
    at Object.urlResolve [as resolve] (url.js:637:40)
    at /app/node_modules/react-snapshot/lib/Crawler.js:96:44
    at Array.forEach (native)
    at /app/node_modules/react-snapshot/lib/Crawler.js:88:83
    at Array.forEach (native)
    at Crawler.extractNewLinks (/app/node_modules/react-snapshot/lib/Crawler.js:86:36)
    at /app/node_modules/react-snapshot/lib/Crawler.js:68:15

This PR just makes the Crawler ignore it and continue crawling.

[geelen]

Closing in favour of #20