improvement: find usages of custom element

[atsu85]

Currently when i press Alt+F7 on custom element (start or end tag name) in html, it shows start and end location of the same tag in same file only. It doesn't even show the usages of other custom elements of the same type in the same file.
It would be nice, if the results view would contain all usages of the same custom element across the whole project.

[denofevil]

Most likely it would after #15 is resolved