Extract helper in spec to set up refactor

This will make it easier to refactor the implementation to use
highlight decorations without changing the tests
Use event-kit, not emissary
Use CommandRegistry::dispatch in spec
Use Workspace::observeTextEditors
Use highlight decorations

It looks like themes that style the bracket-matcher highlight will need
to change.
Remove use of editor-view
Add .bracket-matcher to highlight regions for backward compatibility

We should add this to the linter: atom/atom#4045

Signed-off-by: Max Brunsfeld <maxbrunsfeld@gmail.com>
Merge pull request #82 from atom/mb-use-highlight-decorations

Use highlight decorations