Add Clojure support

The current implementation treated Clojure as an alias of Lisp, which is not
good enough. This commit adds Clojure support based on the rules at
[xzj / clojure.ctags](https://gist.github.com/xzj/1518834), which need
to be added to ~/.ctags. For reference, these are:

```
--langdef=Clojure
--langmap=Clojure:.clj
--regex-clojure=/\([ \t]*create-ns[ \t]+([-[:alnum:]*+!_:\/.?]+)/\1/n,namespace/
--regex-clojure=/\([ \t]*def[ \t]+([-[:alnum:]*+!_:\/.?]+)/\1/d,definition/
--regex-clojure=/\([ \t]*defn[ \t]+([-[:alnum:]*+!_:\/.?]+)/\1/f,function/
--regex-clojure=/\([ \t]*defn-[ \t]+([-[:alnum:]*+!_:\/.?]+)/\1/p,private function/
--regex-clojure=/\([ \t]*defmacro[ \t]+([-[:alnum:]*+!_:\/.?]+)/\1/m,macro/
--regex-clojure=/\([ \t]*definline[ \t]+([-[:alnum:]*+!_:\/.?]+)/\1/i,inline/
--regex-clojure=/\([ \t]*defmulti[ \t]+([-[:alnum:]*+!_:\/.?]+)/\1/a,multimethod definition/
--regex-clojure=/\([ \t]*defmethod[ \t]+([-[:alnum:]*+!_:\/.?]+)/\1/b,multimethod instance/
--regex-clojure=/\([ \t]*defonce[ \t]+([-[:alnum:]*+!_:\/.?]+)/\1/c,definition (once)/
--regex-clojure=/\([ \t]*defstruct[ \t]+([-[:alnum:]*+!_:\/.?]+)/\1/s,struct/
--regex-clojure=/\([ \t]*intern[ \t]+([-[:alnum:]*+!_:\/.?]+)/\1/v,intern/
--regex-clojure=/\([ \t]*ns[ \t]+([-[:alnum:]*+!_:\/.?]+)/\1/n,namespace/
```