Ignore unmatched files option

[Yuhei Yasuda]

I'd like an option to ignore files that do not match the pattern.

// Ignore files other than markdown
metalsmith.use(ignore({ patterns: '**/*.md', unmatch: true }));

[Andrew Goodricke]

Happy for a PR for this feature.

[Yuhei Yasuda]

I am not satisfied with unmatch. Do you have ideas for better name?

[Andrew Goodricke]

invert maybe.
Do you know you can do the following to invert your match

// Ignore files other than markdown
metalsmith.use(ignore({ patterns: '!**/*.md' }));

[Yuhei Yasuda]

Wow, I did not know that😓

I think that it is better to use it.