Added bottom Error Panel

Hi Railk!

Good work on the plugin. I've been keeping quiet for a while and it looks like the intertubes have chosen your plugin as the standard. There are a few things that I really want to improve and I'd like to spend a little time over the next month or two adding some pull requests. 

The first thing is adding an error panel at the bottom that shows all the project errors. I know the error panel you have implemented lets you jump to specific errors, but I find it very hard to read. It is also impossible to keep working while you leave the panel open. 

I haven't changed any of the default functionality yet. The only way to access this is through the command interface: "Tools -> Command Palette -> Typescript: Show Error Panel"

Next steps: 

- If open, it would update automatically whenever errors changed. 
- I think it would be best to replace yours with this, including the key command
- have an option to display this panel automatically on save