Update docs and tests

Author: NigoroJr

doc/zplug.txt

@@ -221,13 +221,18 @@ the `$PATH`, you can run that command from anywhere just like any other commands
 'ZPLUG_USE_CACHE'::
     Defaults to `true`.
     If this variable is true, zplug uses a cache file to speed up the loading process.
-    The cache file is located as `$ZPLUG_HOME/.cache`.
+    The cache file is located at `$ZPLUG_CACHE_FILE` (`$ZPLUG_HOME/.cache` by default).
     If you want to clear the cache, please run `zplug clear` or do the following:
 
 ------------
 $ ZPLUG_USE_CACHE=false zplug load
 ------------
 
+'ZPLUG_CACHE_FILE'::
+    Defaults to `$ZPLUG_HOME/.cache`.
+    Specifies where to save the cache. For example, you can set this to
+    `$HOME/.cache/zplug/cache` to follow XDG Base Directory Specification
+
 
 External commands
 -----------------

test/autoload/commands/__load___test.zsh

@@ -31,6 +31,16 @@ describe "__load__"
         actual="$(zplug load --verbose | unansi)"
         assert.equals "$expect" "$actual"
     end
+
+    it "loads from cache at a custom location"
+        export ZPLUG_USE_CACHE=true
+        export ZPLUG_CACHE_FILE=$ZPLUG_HOME/foo/cache
+
+        mock_as_plugin "foo/bar"
+        expect="Static loading..."
+        actual="$(zplug load && zplug load --verbose)"
+        assert.equals "$expect" "$actual"
+    end
 end
 
 : after