Raise exception if template has invalid variables

If the template string contains variables which aren't valid for whipper, raise ValueError exception listing all the included unrecognized variables.
I've also corrected the example template configuration lines in the README.

Fixes #279.

README.md

@@ -250,7 +250,7 @@ read_offset = 6			; drive read offset in positive/negative frames (no leading +)
 unknown = True
 output_directory = ~/My Music
 track_template = new/%%A/%%y - %%d/%%t - %%n	; note: the format char '%' must be represented '%%'
-disc_template = %(track_template)s
+disc_template =  new/%%A/%%y - %%d/%%A - %%d
 # ...
 ```
 

whipper/common/program.py

@@ -196,6 +196,10 @@ class Program:
         """
         assert isinstance(outdir, unicode), "%r is not unicode" % outdir
         assert isinstance(template, unicode), "%r is not unicode" % template
+        matches = re.findall(r"%[^A,S,d,y,r,R,x,X]", template)
+        if '%' in template and matches:
+            raise ValueError('disc template string contains invalid '
+                             'variable(s): {}.'.format(', '.join(matches)))
         v = {}
         v['A'] = 'Unknown Artist'
         v['d'] = mbdiscid  # fallback for title