add asymmetric header styling option, which most folk use these days

[jonmountjoy]

This creates a new customization variable, markdown-asymmetric-header. Set to non-nil to only have header characters to the left of the title. The default will ensure header characters are placed to the left and right of the title.

For example, if set, headers will be styled like so:

### foo bar

instead of like this

### foo bar ###

[blaenk]

Great!

[vyp]

Thank you @jonmountjoy.

[jrblevin]

Thanks, indeed! Applied just now...

[jonmountjoy]

Cheers!