Provide a semantics-only configuration

[dhutty]

* configure style rules as warnings
* configure rules that can introduce semantic errors as errors

See #72

[coveralls]

Coverage increased (+0.2%) to 97.832% when pulling 75ca77a on dhutty:syntax-only into 734d5d5 on adrienverge:master.

[adrienverge]

Hi @dhutty, thanks for your contribution.

• The rest of yamllint uses the term "syntax", which is also common in other tools. When a syntax problem occurs, it is reported as syntax (rule name). What about syntax-only instead of semantics-only?

• The rule name says "semantics only" or "syntax only", but actually enables most rules (as warnings) and even enables truthy and key-duplicates as errors. Not very understandable for users.

  Instead, what about a syntax-only conf that disables all rules but syntax rule?

  The config file would be simple:

  ---
  
  rules: []

[adrienverge]

I propose to simplify all this to something like:

If you want yamllint to only report syntax errors, use the pre-defined
configuration named ``syntax-only``:


.. code:: bash

 yamllint -d syntax-only file.yaml

[dhutty]

The reason I chose the word 'semantics' was to emphasize that if yamllint exited with an error, it means that the YAML being tested might be semantically different from what the user intended as opposed merely being stylistically/conventionally problematic.

In order words, my proposed yamllint -d semantics-only foo.yaml is a very weak/permissive configuration, which makes most of the common rules about the YAML itself get flagged only as warnings, but anything that could result in broken configuration should cause a failure (i.e. error exit).