Require whitespace before tags in description field

As of v3.0.3 hamster harvests single word #hash tags from the description
field. This PR tweaks the regex used to extract the tags, requiring
whitespace before the # character. This will prevent harvesting a tag from
a pattern like example.com/page#anchor, as mentioned here:
projecthamster#753 (comment).

This does not affect any of the test cases in test_stuff.py nor the example
from the input.page documentation.

src/hamster/lib/parsing.py

@@ -19,6 +19,7 @@
 """, flags=re.VERBOSE)
 
 tags_in_description = re.compile(r"""
+    \s
     \#
     (?P<tag>
         [a-zA-Z] # Starts with an alphabetic character (digits excluded)