implement a way to specify 'extended' format strings

Starting a format string with '\f<Type> ' allows to set a different
format string type than the default. Available ones are

- T: A template file containing the actual format string
     "\fT ~/.templates/booru.txt

- E: An arbitrary Python expression
     "\fE title.upper().replace(' ', '-')"

- M: Name of a Python module followed by a function name inside it.
     This function gets called with the current metadata dict as
     argument and should return a string.
     "\fM my_module:generate_text"

'\f' was chosen since it is highly unlikely that a regular format string
would start with it, but that character could be changed to for example
'\\' or '/' etc.

gallery_dl/formatter.py

@@ -24,8 +24,22 @@ def parse(format_string, default=None):
     try:
         return _CACHE[key]
     except KeyError:
-        formatter = _CACHE[key] = StringFormatter(format_string, default)
-        return formatter
+        pass
+
+    cls = StringFormatter
+    if format_string.startswith("\f"):
+        kind, _, format_string = format_string.partition(" ")
+        kind = kind[1:]
+
+        if kind == "T":
+            cls = TemplateFormatter
+        elif kind == "E":
+            cls = ExpressionFormatter
+        elif kind == "M":
+            cls = ModuleFormatter
+
+    formatter = _CACHE[key] = cls(format_string, default)
+    return formatter
 
 
 class StringFormatter():
@@ -148,6 +162,31 @@ class StringFormatter():
         return wrap
 
 
+class TemplateFormatter(StringFormatter):
+    """Read format_string from file"""
+
+    def __init__(self, path, default=None):
+        with open(util.expand_path(path)) as fp:
+            format_string = fp.read()
+        StringFormatter.__init__(self, format_string, default)
+
+
+class ExpressionFormatter():
+    """Generate text by evaluating a Python expression"""
+
+    def __init__(self, expression, default=None):
+        self.format_map = util.compile_expression(expression)
+
+
+class ModuleFormatter():
+    """Generate text by calling an external function"""
+
+    def __init__(self, function_spec, default=None):
+        module_name, _, function_name = function_spec.partition(":")
+        module = __import__(module_name)
+        self.format_map = getattr(module, function_name)
+
+
 def parse_field_name(field_name):
     first, rest = _string.formatter_field_name_split(field_name)
     funcs = []