Add the readfile function.

README.rst

@@ -117,6 +117,7 @@ Jinja functions
 
 - :code:`run`: Runs a command and returns the stdout, stderr and returncode
   using run_. This function replaces the :code:`run` filter.
+- :code:`readfile`: Returns the contents of a file.
 
 Example usage can be seen in :code:`tests` and for specific filters in the
 docstrings in :code:`template/functions.py`.

template/functions.py

@@ -39,3 +39,21 @@ def run(*argv, **kwargs):
         proc["stdout"] = proc["stdout"].decode()
         proc["stderr"] = proc["stderr"].decode()
     return proc
+
+
+def readfile(path):
+    """
+    Opens a file, returns the contents.
+
+    >>> readfile("/dev/null")
+    ''
+    >>> foo = "foo"
+    >>> with open("/tmp/foo", "w") as f:
+    ...     f.write(foo)
+    3
+    >>> foo == readfile("/tmp/foo")
+    True
+    """
+
+    with open(path, "r") as f:  # pylint: disable=invalid-name
+        return f.read()