add clojure support (string and if-clause)

Added following features for `filetype=clojure`

* string - symbol - keyword switch (like ruby's string - symbol switch)
* if clause switch (like ruby's)

```clojure
"hello-world"
'hello-world
:hello-world
```


```clojure
(if boolean-value
  something
  otherwise)

(if (or true boolean-value)
  something
  otherwise)

(if (and false boolean-value)
  something
  otherwise)
```

vimrunner test passed on my environment. It may depend on your `ftdetect` and `ftplugin`, since this spec depends on your `&iskeyword` value which is usually set by `&lisp`.

I didn't add sentences in README and doc/switch.txt about these new features yet. If you think this pull request satisfies your requirements, I'm happy to write docs.