Use map-stream directly, instead of event-stream

gulp-htmlmin doesn’t need to use event-stream, because es.map() is just
an alias of map-stream.

https://github.com/dominictarr/event-stream/blob/480c38d08f5dffa34c41ac7
37456a3ea98f58d04/index.js#L12
Fix dotfiles

* JSHint supports `mocha` option.
* We don’t need to specify .DS_Store etc. in .npmignore.
* Node v10.0 hasn’t been released yet.
Use SVG badges instead of PNG badges

SVG badges look beautiful on retina displays.