Add target_directory linter setting

This allows the user to specify where to put generated class files.
If unset, class files will be put in the same directory as their
source files.

Closes #1.

README.md

@@ -180,6 +180,19 @@ You can also add whitespace between classpath entries for better readability:
 :/path/to/project/libs/lib.jar
 ```
 
+### `target_directory`
+
+This setting tells `scalac` where to put generated class files. If unset, class
+files will be put in the same directory as their source files.
+
+For example:
+
+```json
+"scalac": {
+    "target_directory": "$PROJECT_PATH/target/scala-2.11/classes"
+}
+```
+
 ## Contributing
 If you would like to contribute enhancements or fixes, please do the following:
 

linter.py

@@ -41,7 +41,8 @@ class Scalac(Linter):
     defaults = {
         'lint': '',
         'classpath': '',
-        'classpath_filename': ''
+        'classpath_filename': '',
+        'target_directory': ''
     }
     inline_settings = ['classpath', 'classpath_filename']
     inline_overrides = 'lint'
@@ -117,6 +118,11 @@ def cmd(self):
         if classpath:
             command += ['-classpath', classpath]
 
+        target_directory = settings.get('target_directory')
+
+        if target_directory:
+            command += ['-d', target_directory]
+
         return command
 
     @property