Strip BOM when reading UTF-8 files

When using gulp-concat, I noticed that the BOM wasn’t stripped from files. When concatenating files together there would be multiple BOMs in the resulting file, which only leads to increased file size for no good reason. (Of course, people shouldn’t use UTF-8 _with BOM_ in the first place, but that’s a separate issue…)

Let’s do what Grunt does and strip BOM by default. Grunt has a setting to override it but IMHO this is not needed. https://github.com/gruntjs/grunt/blob/057eb1c3a7ffdb26b6e2e1a5eb67b262cdd3a71d/lib/grunt/file.js#L238-L241

Thanks to @phated for the pointer to vinyl-fs on IRC.