Settings based docs

These changes will:

   * Add menu options for the key bindings and settings files in Preferences -> Package Settings -> GotoDocumentation
   * Has a dictionary defined at the top of the file with the default docs
   * Merge the default docs with the ones defined in the settings file
   * Adds the `%(query)s` and `%(scope)s` placeholders
   * Adds the option to default to google in case the scope is not defined
   * If a scope has a the value false it will consider the scope disabled and it will not perform a search


Tested on:
   * ST3 - Win7 x64
   * ST2 - Win XP x32
It occurs to me that this could be rewritten as a doc scope of its own. Then instead of a special branch here we just do:

```
if not scope in docs:
    scope = settings.get('fallback_scope', 'google')
if not scope in docs:
    return

# continue as written from here...
```

That'd unify some things, and make it so we need less special-cased code.