Add INTRASITE_LINK_REGEX to configuration that user can modify

This INTRASITE_LINK_REGEX is a string. It should have the capturing
group name which is `what`.

This change was made after discussions with @ametaireau and
@justinmayer.

1. https://github.com/getpelican/pelican/pull/1067
1. https://github.com/getpelican/pelican/pull/1071

Updates getpelican/pelican#1061

pelican/contents.py

@@ -182,13 +182,15 @@ class Content(object):
         if not content:
             return content
 
-        hrefs = re.compile(r"""
+        instrasite_link_regex = self.settings['INTRASITE_LINK_REGEX']
+        regex = r"""
             (?P<markup><\s*[^\>]*  # match tag with src and href attr
                 (?:href|src)\s*=)
 
             (?P<quote>["\'])      # require value to be quoted
-            (?P<path>[|{](?P<what>.*?)[|}](?P<value>.*?))  # the url value
-            \2""", re.X)
+            (?P<path>{0}(?P<value>.*?))  # the url value
+            \2""".format(instrasite_link_regex)
+        hrefs = re.compile(regex, re.X)
 
         def replacer(m):
             what = m.group('what')

pelican/settings.py

@@ -110,6 +110,7 @@ DEFAULT_CONFIG = {
     'TEMPLATE_PAGES': {},
     'IGNORE_FILES': ['.#*'],
     'SLUG_SUBSTITUTIONS': (),
+    'INTRASITE_LINK_REGEX': '[{|](?P<what>.*?)[|}]',
     }