pipeline: Switch to templates for placeholders. #104.

string.format() substitutes all occurrences of {token} with a token in
the formatting map.  Unfortunately, {m,} is also regex syntax for
"match m or more repetitions of preceding regex", and we use {3,} in a
global ignore.

Solution: Use a different delimiter.  Python's string templates look
like they give us enough power to do what we need to do, and they won't
clobber repetition ranges.

Unfortunately, we can't use the default $ delimiter, because $ is a
regex metacharacter.  %# seems sufficiently unlikely to appear in URLs.

db/ignore_patterns/singletumblr.json

@@ -1,7 +1,7 @@
 {
     "name": "singletumblr",
     "patterns": [
-        " ^http://(?!({primary_netloc}|\\d+\\.media\\.tumblr\\.com|assets\\.tumblr\\.com)).*"
+        " ^http://(?!(%#{primary_netloc}|\\d+\\.media\\.tumblr\\.com|assets\\.tumblr\\.com)).*"
     ],
     "type": "ignore_patterns"
 }