Support EditorConfig for configuration

[ejball]

It would be great if we could configure markdownlint in an .editorconfig file. The syntax could be very similar to the JSON configuration, e.g. the options.config example could look something like this:

[*.md]
markdownlint.default = true
markdownlint.MD003.style = atx_closed
markdownlint.MD007.indent = 4
markdownlint.no_hard_tabs = false
markdownlint.whitespace = false

[DavidAnson]

The proposed configuration would be specific to markdownlint - and the VS Code extension as well as both CLI's already support .markdownlint.json/.markdownlint.yaml formats for this purpose. Adding another file and format across the tooling feels like it adds more complexity than it's worth. Thanks for the suggestion!