Named rule sets/style files

[mivok]

Provide support for user-configurable sets of rules that are to be checked by default. These rule sets can specify tags to include, tags to exclude, and rules to include/exclude. This will allow simple configuration of rules to enable/disable to match specific styles, as well as allowing markdownlint to provide some defaults for various styles.

[mivok]

I already use the term ruleset to refer to a given rules file that defines the rules themselves, so for the set of rules that will actually be checked against I'm using the term 'style', as in a particular markdown style that would be conformed to.