Multiline support and switching any blocks

I've been using SublimeERB for some time now. Thank you so much for making this awesome plugin!

Although, I've found few things that bothered me all the time and finally found time to fix them. Here's the list:

- Multiline support. Your plugin simply assumes that there's a single selection all the time, but sometimes it is not the case and you need to make changes on all the selections.

- Changing block to the next one. In your plugin it is possible to change the newly created block, but once you move cursor of type something — you can not change it anymore. So I've fixed that and now you can place your cursor inside any erb block and change it to the next one.

I'm afraid I had to rewrite most of your code to make this happen, so I hope you won't get offended.

The good news is that the plugin works exactly like you made it in the first place, but has many enhancements.