Update feature comparison table

[selfthinker]

Some of the facts in the feature comparison table were outdated. This updates them:

• WAVE and HTML_CodeSniffer are still active while AChecker is not (last updates to WAVE: 28 Nov 2017, HTMLCS: 12 Sep 2017 and AChecker: 6 Sep 2013)
• SortSite can be used internally and behind authentication when using the downloadable software
• SortSite is not completely free but offers a free trial
• Add costs to all free trials
• WAVE CI has a free trial, clarify what you get for $10
• Update and re-phrase CI costs so they say "from (the lowest cost)" to reflect that tools usually have different types of pricing

I also added links to tools in the result table on the start page and adjusted the following URLs to the tools:

• Used axe-core.org for aXe as it is user-friendlier than the GitHub page
• Removed URL parameters from EIII as it is not working
• Also removed parameters from Nu as it would be the only one left
• Changed Nu to use w3.org URL (which is the same backend)