Add flag g:commentary_map_backslash

- If `g:commentary_map_backslash == 0`, don't map the old
  backslash-prefixed normal commands

The `\\`, `\\\`, and `\\u` maps bother me, as someone who uses `\` as his `mapleader`. They don't get mapped if maps already exist for those keys, but I don't want them there at all, ever. This adds a flag to allow for that. Default behaviour is unchanged.