Filter gitignore'd files

[borekb]

As initially discussed in #44, it would be great if markdownlint could pic up ignore rules from .gitignore files. With a flag suggested by @DavidAnson, it could look like this:

markdownlint --gitignore

(I'd prefer to put it in a config file but that is a generic request, see #45.)

[maxmilton]

I quite like ESLint's --ignore-path flag, e,g,:

eslint --ignore-path .gitignore .

It also follows linter conventions of supporting a .eslintignore file. Would be cool to support a .markdownlintignore file too since it would likely use the same logic as parsing and loading from .gitignore.

[borekb]

I now also prefer what @maxmilton said, for example, for ESLint, I can do:

eslint --ignore-path .gitignore --ignore-path .prettierignore ...

which is actually what I'd use for markdownlint as well.

[DavidAnson]

With the commit for #61, .markdownlintignore is now supported as with .eslintignore. Support for --ignore-path could use this to allow people to point at different files, but I'm hopeful that .markdownlintignore is the right choice most of the time.