[Feature] Allow filtering for accounts

[putzwasser]

TLDR
Allow filtering accounts to exclude specific assets from fava investor modules

---

Say you have the following asset_alloc config

2000-01-01 custom "fava-extension" "fava_investor" "{
  'asset_alloc_by_class' : {
    'accounts_patterns'    : [
			'Assets:Stocks',
			'Assets:Bonds',
			'Assets:Cash',
			'Assets:RealEstate'],
  },
}"

RealEstate will make probably 70-95% of your asset allocation. While this is good to know and good to track, it would be nice to exlucde the real estate account from the view via the fava webUI. (Filter: -Asset:RealEstate). This would allow to easily check the allocation of your more liquid assets and track the asset allocations on "every level", without the need to adjust the config and restart fava.

[redstreet]

Sounds like a useful feature to have. What happens when the negative filter is used currently? Is it just a simple patch to not treat missing accounts as an error? If so, patch+tests welcome.