add joker handler for clojure

Adds new linter for clojure using joker

https://github.com/candid82/joker

fixes #975
ref #544 #1040

doc/ale-clojure.txt

@@ -0,0 +1,21 @@
+===============================================================================
+ALE Clojure Integration                                   *ale-clojure-options*
+
+
+===============================================================================
+joker                                                       *ale-clojure-joker*
+
+Joker is a small Clojure interpreter and linter written in Go.
+
+https://github.com/candid82/joker
+
+Linting options are not configurable by ale, but instead are controlled by a
+`.joker` file in same directory as the file (or current working directory if
+linting stdin), a parent directory relative to the file, or the users home
+directory.
+
+see https://github.com/candid82/joker#linter-mode for more information.
+
+===============================================================================
+  vim:tw=78:ts=2:sts=2:sw=2:ft=help:norl:
+

doc/ale.txt

@@ -29,6 +29,8 @@ CONTENTS                                                         *ale-contents*
       gcc.................................|ale-c-gcc|
     chef..................................|ale-chef-options|
       foodcritic..........................|ale-chef-foodcritic|
+    clojure...............................|ale-clojure-options|
+      joker...............................|ale-clojure-joker|
     cmake.................................|ale-cmake-options|
       cmakelint...........................|ale-cmake-cmakelint|
     cpp...................................|ale-cpp-options|
@@ -271,6 +273,7 @@ Notes:
 * CUDA: `nvcc`!!
 * C#: `mcs`, `mcsc`!!
 * Chef: `foodcritic`
+* Clojure: `joker`
 * CMake: `cmakelint`
 * CoffeeScript: `coffee`, `coffeelint`
 * Crystal: `crystal`!!