Allow disabling of catch-all page matching in routes

Adds an initializer config setting 
```
Cms.match_nonexisting_routes = true
```
That when set to false will disable the broad path match in route_extensions.rb
```
match "*path", :to=>"cms/content#show"
```
This has the effect of allowing mount_browsercms to be mounted in locations other than the bottom of routes.rb in the existing app.  This can be desirable in several situations such as when it's necessary to use the existing app's error handling rather than BCMS.