Add prefer-text-content rule

[alexzherdev]

Resolves #156.

[sindresorhus]

Hey, thanks for the pull request 🙌 I have gotten a huge response on the IssueHunt bounty issues and I now have a very long backlog of pull requests to review, so it might take some time until I will be able to review this. However, you can help move this along by helping out and reviewing the other submissions. If everyone helps everyone we can manage this 👌.

Some things to look for when reviewing:

• Ensure the rule addition adheres to the instructions.
• There are never too many tests. Try to find both passing and failing cases, especially obscure cases.
• Check the docs for typos and whether something could be more clear.
• Look at the rule code and see if anything could be simplified or done better. If something is unclear in the code, ask the submitter to explain.

[sindresorhus]

Can you also add:

• innerText = true;
• 'node['innerText'];'

[sindresorhus]

Can you also add an assignment case: node.innerText = 'foo';

[sindresorhus]

I just realized that to be consistent with the naming of the other rules, this rule needs to be named prefer-textcontent => prefer-text-content.

[sindresorhus]

Nice work, @alexzherdev 🙌