Heading mods

Here's a quick pass at adding the behavior specified in this issue https://github.com/GitbookIO/kramed/issues/4

This idea is to
  * *replace* the header ID with the specified one instead of concatenating
  * remove the trailing hyphens in resultant ID
  * remove the attribute block {#...} from the resulting output

I'm fairly new to regex, so I'd expect that this could be made more robust. 

This approach fails on more than one `#ID` in the atrribute block `{#ID}`

Wishlist for these mods would be to match this behavior https://michelf.ca/projects/php-markdown/extra/#spe-attr so there can be multiple IDs and Classes passed through the attribute block. It'll probably be slower, so this can be called '**Kramed, built for speed***'