Improve Python API

[verhovsky]

I'm using yamllint from Python because my YAML is frontmatter in Markdown files, so I have to parse it out before linting it and I have to say the API is bad/unpythonic. I would expect this

import yamllint

yamllint.lint("foo: bar")

to work and use the default config, and I should not be required to make a YAMLConfig object (from a string!), I should be able to just pass a dictionary

import yamllint

yamllint.lint("foo: bar", "relaxed")

yamllint.lint("foo: bar", {"extends": "relaxed", "rules": {}})

also, https://yamllint.readthedocs.io/en/stable/development.html is still outdated after #699 which wasted some of my time and it also doesn't document all the properties of LintProblem. "Development" is also a confusing name for this section because "Development" in documentation usually refers to how to contribute to the underlying project, not how to use it. Also the readme should mention that there is a Python API, because I couldn't tell.