Explicit insert mode

Supersedes #256.

Before, we used to pass all keystrokes through if an editable element happened to be focused. This allows you to type into inputs, textareas, the location bar etc. It has one downside, though: If an input is autofocused and you don’t notice it, it will feel like VimFx does not work, because no commands work. Instead there will be lots of 'j':s and 'f':s etc in a text input somewhere.

With this pull request, we don’t pass keystrokes through like that anymore. If you’re in normal mode, your keystrokes will activate commands. That’s it. No exceptions.

However, that would be inconvenient. If you focus a text input (by clicking it, using the 'f' command, tabbing to it), of course you want to type into it. You don’t want to manually enter insert mode first. So in those cases, we automatically enter insert mode, with `{auto: true}`. With that flag, we automatically go back to normal mode when you blur the input.

The above also resulted in, IMO, much nicer code.

But what about the case where you actually _want_ autofocus? Easy: Just press 'i' and you’re ready to type!

This makes insert mode more explicit. First off, the actual insert mode is used when you type into input, instead of just passing keystrokes through. Secondly, insert mode is only entered when _you_ (not the developer who didn’t think of keyboard users and added `autofocus` to the search box present on every page of the site) want to.

There is one semi-resolved thing though: autofocused inputs still look focused, with a blinking caret. But you can’t type into it. We have experienced with styling the input in that case, to show that it isn’t _really_ focused. The only way to see this currently is by looking at the VimFx icon (being green and not grey). But you don’t really have to think about it, because you know if you focused the input yourself or not.

Also, I just thought about another "issue" related to the above: Letters that have no command (such as 'c') _will_ show up in autofocused inputs. Usually, this is a feature: Letting you use some site shortcuts at the same time as you use VimFx. But, in this case, it is just weird. Perhaps we should suppress letter keypresses in that case.