fix for issue #13

added new global variable *g:XkbSwitchSkipGhKeys*, which can be used to
disable *gh*-mappings on your own. For example

```vim
let g:XkbSwitchSkipGhKeys = ['gh', 'gH', 'g^H']
```

disables all *gh*-mappings. Beware that '^H' in the 3rd element is a
single ASCII symbol and is not composed from '^' and 'H"!