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]

:+1: