Loosen selector to class names to allow integration
Loosen selector to class names to allow integration

`[is=tree-view-file]` enables only the custom element that registered as
`tree-view-file` to integrate with this package's preview tabs feature.
Using `.file-tree .entry.file` allows other file tree implementations to
match the selector without having to commandeer the globally-unique
"tree-view-file" custom element name.

Fixes #208

atom/tabs