added KeyBindings

[mohan-mu]

Hi Maintainers, I have added KeyBindings for the below keys

#71 ,#50
Ctrl-Alt-1 : Heading 1
Ctrl-Alt-2 : Heading 2
Ctrl-Alt-3 : Heading 3
Ctrl-Alt-4 : Heading 4
Ctrl-Alt-5 : Heading 5
Ctrl-Alt-6 : Heading 6
Shift-Ctrl-L : Links,
Shift-Ctrl-I : Image,
Shift-Ctrl-. : BlockQuote
Shift-Ctrl-':CodeBlock
Shift-U: Unordered list,
Shift-O :Ordered list,

Please check it out

Thank You

[aero31aero]

Hey, I'm sorry for a late review on this.

I've pointed out two main issues I see with the current code and I think it'd be trivial to fix these.

Also, I think we can safely use the selectionChanger function for headings like:

cm.replaceSelection(
    selectionChanger(
        cm.getSelection(), '### ', ''
    )
);

Thanks for the changes @mohan-mu.

[aero31aero]

This change takes away the existing functionality where you could use CTRL+B to toggle bold. This would always add more text. For example, pressing the hotkey twice on 'hello' would result in:

=> hello
=> **hello**
=> ****hello****

Arguably, the first approach wasn't foolproof either, but this implementation is immediately broken.

[aero31aero]

Your changes appear to change the code to be indented with 2 characters instead of 4, which is the current convention of the file. I'd appreciate if you could make sure the file remains consistent.