Automatically detect character encoding of YAML files and ignore files

[Jayman2000]

This PR makes sure that yamllint never uses open()’s default encoding. Specifically, it uses the character encoding detection algorithm specified in chapter 5.2 of the YAML spec when reading both YAML files and files that are on the ignore-from-file list.

There are two other PRs that are similar to this one. Here’s how this PR compares to those two:

• This PR doesn’t have any merge conflicts.
• This PR has a cleaner commit history. You can run the tests and flake8 on each commit in this PR, and they’ll report no errors. I don’t think that you can do that with #240.
• This PR has longer commit messages. I really tried to explain why I think that my changes make sense.
• This PR detects the encoding of files being linted, config files, and files on the ignore-from-file list. Those two PRs only detects the encoding of files being linted.

• 240 PR adds a dependency on chardet. This PR doesn’t add any dependencies.

• This PR only supports UTF-8, UTF-16 and UTF-32. Both of those PRs support additional encodings.

• 581 adds support for running tests on Windows. This PR doesn’t.

• The code that this PR adds to the yamllint package is simpler.
• The code that this PR adds to the test package is much more complicated, but hopefully it tests things more thoroughly.

Fixes #218. Fixes #238. Fixes #347. Closes #240. Closes #581.

[coveralls]

coverage: 99.835% (+0.01%) from 99.825% when pulling aeabade02eebc30a0e2937224af7f26f0a9ce0bf on Jayman2000:auto-detect-encoding into f0c0c7586b97809289bdcfe83a66363330b645d8 on adrienverge:master.

[Jayman2000]

I just noticed that one of the checks for this PR is failing. The coverage for yamllint/config.py went down, but that’s just because the total number relevant lines went down. There’s only two lines that aren’t covered, but those same two lines aren’t covered in the master branch. Is there anything that I need to do here?

[adrienverge]

Is there anything that I need to do here?

At the moment, no. I'm sorry, please excuse the delay, this is a big change with much impact, I need a large time slot to review this, which I couldn't find yet.