Highlight occurences of selected word

When you:
- double click a word
- trigger `find-and-replace:select-next` command (`Cmd+d` by default) it will
select the word under the cursor. Now it will also highlight all the other
occurences of the word in the file.

Partially addresses #138.

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This is working, but it is not the complete feature from Sublime Text.

The activation events thing in Atom is nice, but it could be quite limiting.

Right now the double-click action is not triggered until the package has not
been activated in another way.