Automatically focus on text input when the window gets focus

[knu]

Without this, you always have to click on the text input after switching back from another application where you've copied something you intend to paste to your tweet.

[eramdam]

1. I won't merge the PR until you fix the lint issues (see the Travis build ). Most of them are indentation-related so you should be able to fix them by running the following command
   ./node_modules/.bin/eslint --fix src
2. I guess this will fix #89 ?

[knu]

Oops, sorry, I just pasted my local userscript and completely missed the style issues.
Just updated the commit.

[knu]

I've fixed a typo and locally tested the extension again.

[eramdam]

@knu FYI: I had to tweak your code a bit. You can see the commit here fc3f265.

Here is what I did:

1. I improved the way you detect the visibility of an element, it was only using height and width. It could have worked except the <textarea> element of the tweet composer always have non-zero height/widths, it's just partially masked. I simply used getBoundingClientRect() to see if one of the top/bottom/right/left values was negative or not
2. You listener was triggering on every focus events happening on the page, messing up with other features of TweetDeck. I added a simple check to see if the target of the event is the window or not.

[knu]

@eramdam Great, thanks!