handle non-intable strings better

[thingscouldbeworse]

Getting errors today of the shape

Traceback (most recent call last):
  File "/home/user/.local/lib/python3.8/site-packages/billboard.py", line 374, in getMeta
    return int(value)
ValueError: invalid literal for int() with base 10: ''

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  ...
  File "/home/user/.local/lib/python3.8/site-packages/billboard.py", line 190, in __init__
    self.fetchEntries()
  File "/home/user/.local/lib/python3.8/site-packages/billboard.py", line 489, in fetchEntries
    self._parsePage(soup)
  File "/home/user/.local/lib/python3.8/site-packages/billboard.py", line 463, in _parsePage
    self._parseNewStylePage(soup)
  File "/home/user/.local/lib/python3.8/site-packages/billboard.py", line 382, in _parseNewStylePage
    weeks = getMeta("week", 5, ifNoValue=1)
  File "/home/user/.local/lib/python3.8/site-packages/billboard.py", line 377, in getMeta
    raise BillboardParseException(message)
billboard.BillboardParseException: Failed to parse metadata value: week

Seems like the fix in this PR is the easiest, just return our ifNotValue for anything that isn't intable.