introduce .courseignore file

You can now place a file name '.courseignore' in the ROOT directory
and specify - in each line - the name of a directory. Directories named
this way and located in the ROOT directory will not be treated as course
directories

.gitignore

@@ -1,2 +1,3 @@
 *.swp
 scripts/__pycache__
+scripts/test.py

scripts/courses.py

@@ -29,10 +29,18 @@ class Courses(list):
         list.__init__(self, self.read_files())
 
     def read_files(self):
-        course_directories = [x for x in ROOT.iterdir() if x.is_dir()]
+        course_directories = [x for x in ROOT.iterdir() if x.is_dir() and not x in self.ignored_courses()]
         _courses = [Course(path) for path in course_directories]
         return sorted(_courses, key=lambda c: c.name)
 
+    def ignored_courses(self):
+        with open(ROOT / '.courseignore') as ignore:
+            lines = ignore.readlines()
+            paths = []
+            for line in lines:
+                paths.append(ROOT / line.strip())
+            return paths
+
     @property
     def current(self):
         return Course(CURRENT_COURSE_ROOT.resolve())