crash when using tags

[Akuukis]

Reproduce: use #split-month and see fava burn. Problem line: tx.meta.pop('split'). I guess we need to check if split exists in meta first, as well clean the tags also.

Stacktrace

Exception on / [GET]
Traceback (most recent call last):
  File "$HOME/.local/lib/python3.8/site-packages/flask/app.py", line 2447, in wsgi_app
    response = self.full_dispatch_request()
  File "$HOME/.local/lib/python3.8/site-packages/flask/app.py", line 1952, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "$HOME/.local/lib/python3.8/site-packages/flask/app.py", line 1821, in handle_user_exception
    reraise(exc_type, exc_value, tb)
  File "$HOME/.local/lib/python3.8/site-packages/flask/_compat.py", line 39, in reraise
    raise value
  File "$HOME/.local/lib/python3.8/site-packages/flask/app.py", line 1948, in full_dispatch_request
    rv = self.preprocess_request()
  File "$HOME/.local/lib/python3.8/site-packages/flask/app.py", line 2236, in preprocess_request
    func(request.endpoint, request.view_args)
  File "$HOME/.local/lib/python3.8/site-packages/fava/application.py", line 234, in _pull_beancount_file
    _load_file()
  File "$HOME/.local/lib/python3.8/site-packages/fava/application.py", line 101, in _load_file
    ledger = FavaLedger(filepath)
  File "$HOME/.local/lib/python3.8/site-packages/fava/core/__init__.py", line 175, in __init__
    self.load_file()
  File "$HOME/.local/lib/python3.8/site-packages/fava/core/__init__.py", line 182, in load_file
    self.all_entries, self.errors, self.options = loader._load(
  File "$HOME/.local/lib/python3.8/site-packages/beancount/loader.py", line 476, in _load
    entries, errors = run_transformations(entries, parse_errors, options_map,
  File "$HOME/.local/lib/python3.8/site-packages/beancount/loader.py", line 551, in run_transformations
    entries, plugin_errors = callback(entries, options_map,
  File "$HOME/.local/lib/python3.8/site-packages/beancount_interpolate/split.py", line 6, in split
    return interpolate_subset(entries, options_map, {'split': config_string})
  File "$HOME/.local/lib/python3.8/site-packages/beancount_interpolate/interpolate.py", line 63, in interpolate_subset
    entries, command_errors = commands_to_functions_map[command](entries, config_map[command])
  File "$HOME/.local/lib/python3.8/site-packages/beancount_interpolate/interpolate.py", line 201, in split
    tx.meta.pop('split')
KeyError: 'split'

[Akuukis]

Fixed by #14