Also check for .tags

This PR would theoretically make the package scan for `.tags` as a possible source in addition to `tags` and `TAGS`. It'd be useful to have the ctags file hidden to make the root a bit cleaner. As well as that, convention in the Sublime Text [CTags](https://github.com/SublimeText/CTags) package is to put the ctags file as `.tags`.

I use "theoretically" since the PR doesn't work yet. By modifying the package in Atom.app itself, adding:

      tagsFile = atom.project.resolve(".tags");
      if (fs.isFileSync(tagsFile)) {
        console.log('hi');
        return tagsFile;
      }

It looks like it can find `.tags`, but for some reason still says "Project has no tags file or it is empty". Trying to work with the package in dev mode triggers one of these:

    Uncaught Error: Cannot find module 'pathwatcher' <- cmd+shift+r
    Uncaught Error: Cannot find module 'fs-plus' <- cmd+r, alt+cmd+up
    Uncaught Error: Cannot find module 'q' <- alt+cmd+down