A few vim improvements

[jtratner]

4 changes:

1. check to see if xiki_dir is defined first, then try environment
   variable and finally use 'xiki directory'
2. allow mappings to be skipped if g:XikiUseDefaultMappings is set to
   0
3. Only load the plugin once (though not really a big deal for the size
   it is now)
4. export the function XikiLaunch() as the command XikiLaunch

[keithtom]

👍