Strip BOM when reading 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.

Let’s do what Grunt does and strip BOM by default. Grunt has a setting to override it but I don’t think this is 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.